Whatever your motivation, a few simple upgrades can make your PC more productive. And these upgrades make more sense than an all-new PC, especially with CES and other tradeshows coming up.
RAM: This is easily the most important upgrade you can make, especially if you short-changed your hardware initially. You should have at least 2GB, but shoot for about 4GB.
The 32-bit versions of XP and Vista cap the memory usage there and might not even be able to address all of it. (64-bit Windows can use more than 4GB.) Still, RAM is cheap; your programs will run more smoothly, and you'll be able to have more open at a time.
Consult your system documentation to see what kind of memory it takes and if you'll benefit from installing it in pairs. Otherwise, shopping sites often list specific PC models.
Hard drives: You won't get a noticeable performance boost out of a hard drive upgrade - unless you've been using your PC with only a few GB free - but you'll increase the storage space. Hard drives are a great way to manage backups, too, since they're fairly cheap. I recently swapped my original 120GB laptop drive with a 320GB disk after my music and multiple OS partitions became too crowded.
Video card: Depending on how you use your PC - and the age of the old card - a new video card might give a major performance boost. A new card could handle higher resolutions than your old one, which is useful for adding bigger or extra displays. And while the market adoption is still growing, some software, such as Photoshop, can offload tasks unrelated to video to those graphics processors. Throughout 2009, that trend should continue, supplementing CPUs in more ways.
Processor: This is lower on my universal list because most business tasks aren't too CPU-intensive. That said, if your processor is ready for retirement and you have options that will fit your motherboard socket, this upgrade can make a significant impact for a couple hundred dollars.
Displays: Consider new, flat-panel displays, especially if you're still using CRTs. More screen space can speed up everyday tasks by letting you keep multiple windows visible. Or add a second display if supported by your video card. (Nearly all do.)
Input devices: Finally, if you're still using the pack-in keyboard and mouse, please stop. Treat your hands to even more years of carpel tunnel-free life. I type almost exclusively on mechanical-switch keyboards, which usually run about £60 or so, but feel significantly more comfortable than the £10 rubber-membrane keyboards. Mouse options abound, but I especially like trackballs for huge - or multiple - displays. Pick something that feels good to you.