The next version of Mac OS X will go on sale on Friday 26 October, and Apple is already taking pre-orders. That means Mac users finally have a decision to make.

Is a move to Leopard smart? Everyone will have different reasons for considering the switch, but assuming you give Apple a nod and your £85, here are the questions to ponder as you prepare for next Friday. We'll start with the basics first.

What is Leopard?

Leopard is the code name for Mac OS X 10.5, the latest in a string of operating system releases from Apple that go by names from the family Felidae. The current operating system, Version 10.4, is Tiger, which was preceded by Panther, Jaguar, Puma and Cheetah, in reverse order. It is the first major release - in Apple's eyes, that means a shift in numerical nomenclature of one-tenth of a point - since April 2005. It also marks the longest time between releases since Apple started rolling out Mac OS X.

As to why Apple CEO Steve Jobs and company insist on naming their operating systems after cats, you'd have to ask them. But with 41 species altogether in the family, there are enough to keep going for another 43 years or so.

Mac OS X 'Chinese Mountain Cat', anyone?

Why would I want to spend £85 on a new operating system? Tiger is working fine. We're betting you're taking a subtle shot at Windows Vista here. If not, Apple says it has more than 300 reasons for upgrading, citing the number of new features it claims that Leopard has. Many are under the hood and hard to spot for all but the most dedicated - as in the Apple Dictionary, which the company says now includes "a dictionary of Apple terms." Feature or market-speak - you decide. But there are several that make most everyone's justification list, including these five: Time Machine, Boot Camp, Parental Controls, a revamped Desktop and a more powerful Spotlight search.

Whether the changes are worth the £85 is, of course, between you and your bank account.

Will my Mac run Leopard?

Apple has struck some older PowerPC-based Macs from the upgrade list by requiring at least a PowerPC G4 processor running at 867MHz. That means the 800MHz PowerBook G4, 800-MHz iMac G4, 800-MHz iBook G4 and others of their ilk and age are out of luck, officially anyway. All Intel-based Macs are Leopard-ready, however, as are all PowerPC G5 systems.

Leopard's other requirements are 512MB of RAM, 9GB of free space on the hard drive, and a DVD drive.

I've got three Macs at home. Is Apple selling Leopard in a Family Pack?

Yes. Apple is already taking pre-orders for a five-licence edition, priced at £129. Leopard's Family Pack licence sports the same conditions as Tiger's, which means you can install the operating system on up to five Macs in the same household (machines with kids at college count). The Pack is an unbeatable deal: £26 per machine if you have five Macs.

What should I do before I upgrade?

Back up. Accidents do happen, even on the Mac, and if an upgrade sours, you'll wish you had played it safe. At the least, you'll want to back up data files - your iTunes library, documents, photos and the like - to an external drive, flash drive, CD or DVD. A full backup is even better, and a bootable backup is best. SuperDuper, a free backup program, lets you clone the boot volume of your Mac so that if the Leopard upgrade craps out the Mac, you can boot Tiger from an external drive. Just make sure your backup hard drive is large enough to hold all your files. You know how fast all those vacation pictures can add up.

What's the best way to upgrade from Tiger to Leopard?

Do I have to wipe my hard drive and start from scratch? Leopard allows users to upgrade, archive and install, or erase and install.

Those options will be familiar to experienced Mac users, but for recent switchers from the world of Windows, this will be their debut. In a nutshell, Upgrade is the least intrusive because it leaves most applications and settings intact and untouched as it updates the operating system. It's also the least thorough, as even Apple acknowledges. "If you're having issues with your currently-installed version of Mac OS X, upgrading may not resolve those issues," reads a support document on the Apple site. In other words, if something's broken in Tiger, simply moving to Leopard might not fix it.

Archive and Install also takes a hands-off approach to existing data, applications and settings, although some third-party software may balk or even be broken. With this option, Tiger's system files are moved to a folder dubbed 'Previous System', and Leopard's system files are then copied to the disk. You need more free space on the hard drive for Archive and Install than either of the other options, since you're keeping a copy of Tiger. The Previous System folder can be deleted later once you're certain everything is working smoothly, but you can't use it any longer to start up your computer.

Erase and Install is just what it sounds like: a fresh operating system install that overwrites the existing data, forcing you to reinstall all third-party software and restore data from a backup. If you've made a backup of your Tiger system and files, you can use Apple's Migration Assistant - check your Utilities folder - to copy them back to your new Leopard system in the correct place. If you've made a backup of your Tiger system and files, you can use Apple's Migration Assistant - check your Utilities folder - to copy them back to your new Leopard system in the correct place. The Migration Assistant only works between boot volumes, so the backup (Tiger) has to be a bootable image located either on another partition on your hard drive or accessible via FireWire.

Each option has its proponents, and at times, an argument breaks out. Mac users who spent years, or decades, in the Windows trenches often carry the conditioned habit of a fresh install with them to the Mac. Plenty of people are guffawing in message forums that this is overkill in Mac OS X-land, where the simplest upgrade is perfectly safe in almost every case. "Clean install is basically a Windows mentality," wrote Loge on this MacRumors forum two weeks ago.

Others, such as those who apparently can't make up their minds, take the middle-of-the-road Archive and Install option.

Can I run Windows with Leopard?

Yes. Boot Camp, Apple's dual-boot software that has been available in beta to Mac OS X 10.4 owners free of charge, is built into Leopard. It lets users of Intel-based Macs boot into either Leopard or Windows, or quit one operating system and cold boot into the other. Unlike virtualisation software such as SWsoftware's Parallels Desktop or VMware's Fusion, however, Boot Camp does not run both Windows and Leopard simultaneously.

Boot Camp installs and manages Windows XP Home SP2, XP Professional SP2, Vista Home Basic, Vista Home Premium, Vista Business and Vista Ultimate, so you'll need a retail copy of Windows: licenses tied to a PC aren't transferrable. A full version or a reseller edition, also called a 'system builder' edition, are your only options. You can't install an XP or Vista upgrade stock-keeping unit.

Just a thought: With Vista's additional hardware demands and the knocks it has taken from users, Windows XP seems the consensus pick of Mac owners writing to various message forums, including Apple's. Although Microsoft Corp. recently extended the retail lifespan of XP another five months to June 30, 2008, it's never been tough to find the older operating system - even after Vista's release. Brick-and-mortar stores have it on the shelves, and there's still time to order a copy online before Leopard shows up on the 26th.

I'm already running Boot Camp, so will I have to reinstall Windows too?

No. Boot Camp puts Windows XP or Vista on a separate partition, which remains undisturbed by the upgrade.

I'm running Windows using Parallels Desktop (or Fusion); will I have to do anything special?

This isn't as clear-cut. SWsoftware, for example, said last week that Parallels Desktop works with Leopard. "You'll be able to safely upgrade to Leopard when it goes live without worrying that Parallels will work," said Benjamin Rudolph, director of corporate communications, on the company's blog.

VMware's Fusion, meanwhile, will also work with Leopard, according to a message posted to a support forum by Pat Lee, senior product manager. "VMware Fusion 1.1 is free for all paying 1.0 customers, and it will include support for Mac OS X Leopard," Lee said. However, Fusion 1.1 only recently entered beta testing, and VMware won't talk about a release schedule yet, so you might need to run an unfinished version under Leopard for awhile.

But exactly what users of either virtual machine (VM) will need to do, if anything, before or after a Tiger-to-Leopard upgrade is unknown. Anyone taking the Erase and Install route, of course, will have to reinstall, at a minimum, the VM and then Windows in the VM.

What's the one new piece of hardware that will help strut Leopard's stuff?

A new hard drive. Time Machine, the new feature in Leopard, doesn't require an external drive to do its document-versioning magic, but adding one is the simplest way to back up and restore. (Time Machine will back up to any non-booting drive or partition, including internal and networked volumes, although Apple is pushing the external drive route.) Time Machine supports both FireWire and USB external drives, so you have plenty of choices.

Second on the wish list, and related to Time Machine, is the AirPort Extreme wireless router. Plug an external drive into the AirPort's USB port, and any wireless-enabled Mac can back up files to the disk over the Wi-Fi network. But you'll have to do the backup the old-fashioned way, by dragging files and folders to the disk's icon. Time Machine doesn't work with wireless disks - at least, not yet.

If you plan to go wireless with the AirPort, remember that USB-only drives cost less than multiport models that offer, say, both USB and FireWire interfaces.

And what happens if I want to roll back my operating system to Tiger?

Ah, the dirty secret of operating system upgrades. Not everyone will be happy with Leopard, no matter how much Apple amps up the Jobsian reality-distortion field. But reverting to Tiger, say, after updating from it to Leopard, is a lot like herding cats: possible perhaps, but not very pretty.

This isn't an Apple-only problem. Reversing an upgrade on almost any operating system is a major pain. Just ask anyone who has decided that Vista is valueless to them and hopes to downshift to XP.

Although some might hint that the Archive-and-Install option lets you roll back your operating system, that's a lie. You can't boot from the Previous System folder, nor can you rebless that folder ("bless" in Mac terminology means designating a folder as the boot folder) to force a return to Tiger. The only sure way to return to yesteryear is by planning ahead, which means backing up what you have before you upgrade. You can then wipe the drive and restore the backup -- this is another pitch for bootable backups made by the likes of SuperDuper or Carbon Copy Cloner - to your Mac's drive.

Before you do that, however, you'll want to back up any settings, documents, photos, music and data that you created and/or changed after the Leopard upgrade so they can be restored after the operating system has been.