HOW TO SPICE UP THAT OLD SYSTEM
Other items to consider upgrading are the hard drive, CDROM, sound card, and speakers. In today's desktops, 13+gig hard drives are becoming standard. If you're still chugging by with a 1-5 gig, 3200 RPM drive, then your definitely need to upgrade. I recommend have at least 8 gigs and 5400RPM with 7200RPM being desired. The IBM Desktar systems seem to be good drives and the 14.4 gigger comes highly recommended.
The CDROM drive can be important for some people, unimportant for others. Old CD-ROMs sometimes can't read CDRs or CDRWs and are generally quite slow. If you've got anything less than 12x I would recommend getting a new drive. 40x CD-Roms can be had very cheaply these days. Of course, DVD is an option and if you wanna watch movies on your PC, it can be done for around $100 bucks. CDRW drives are also getting to be very inexpensive. You could pick up a Hewlett-Packard 4x/2x/24x for less than $400 for sure.
Having a good sound card can be nice but I don't really have a lot of experience in that department. I use an Ensonique PCI card that I got with my Gateway P200 and has worked great for me. If you want a good 3d-sound card than the best option are the Diamond MX300 and Creative Labs SBLive Value pack. Of course if you've got a nice sound card you shoud have good speakers. A great bang-for-the-buck deal is the 3-piece Altec Lansing ACS-48. For 4 speaker sets, I believe Cambridge does a good job with their Soundworks speakers.