While toiling around with Windows Vista Service Pack 2 Beta, I took a look at a couple areas where Microsoft emphasizes better efficiency: The Service Pack Cleanup tool and a more efficient Sidebar.
In my hands-on experience, I found improvements to be slight to the point of being almost unnoticeable.
Service Pack Cleanup Tool
Bundled with SP2 Beta is the so-called Service Pack Cleanup Tool, also known as compcln.exe. This little tool removes older versions of system files replaced by Service Pack 2, saving you some valuable hard disk space.
A caveat about the cleanup tool: Once you run it, there's no going back. You can't roll back to Service Pack 1 without erasing your hard drive once the cleanup tool does its thing, so make sure you really want to stay with Service Pack 2 Beta beforehand.
Before running the cleanup tool, I noted my test PC's free hard disk space: 44.4GB. After running the cleanup tool, the drive's free space increased slightly to 44.8GB. However, over the next few minutes, free space actually decreased slightly, to 43.9GB.
To rule out the possibility of virtual memory affecting the disk's free space, I restarted and noted the drive's free space again: 44GB -still slightly below where I stood before running the cleanup tool. After a second restart, free space stood at 45GB. Long story short - you may or may not notice some increase in free space, but don't expect miracles.
Keep in mind, though, that your savings may depend on how long you've had Vista installed on your PC. If you installed Vista a year ago, you may have more previous versions of system files than if you installed it a month ago, you your results may vary.
More efficient Sidebar
Microsoft also states that Sidebar gadgets now use fewer system resources. In my casual, non-scientific testing, I monitored the Sidebar's memory use on two PCs side-by-side: An HP notebook running Vista SP2 Beta and a Dell Optiplex desktop running Vista SP1, both with 2GB of memory.
To ensure that my comparison was as fair as possible, I removed all Sidebar gadgets, restarted, then added the same set of gadgets on both PCs. Over the course of a few hours, I let the two PCs run, and performed the same tasks with the gadgets on both: Adding some, removing some, changing some settings, and poking at them every so often.
In the end, SP2 Beta did show an improvement, though again it was slight. At first, SP2 Beta's Sidebar actually used slightly more memory than SP1's but over time, SP2 Beta showed a slight advantage. Over the course of an afternoon, SP2 Beta's Sidebar used maybe one or two fewer megabytes of memory than its SP1 counterpart.
Mind you, the difference will depend on which widgets you have open, and if you have your computer running for days at a time, it may be more obvious, but in my relatively short-term tests, the difference is negligible.