Thanks everyone for your input.
After some tinkering today, I got it to post! I have three computers currently working now. I had to remove my video card and sound card from my micro ATX computer case (running an Athlon XP 3000+) and fit it into the XP 3200+ machine.
Basically now, the micro ATX case that I was using as a backup gets a crappy sound card and on-board video for the time being. The computer that I couldn't get it to post is now running a 3200+ processor, 1.5gb ddr400, 80gb Seagate harddrive, nVidia 5900XT 128, and a SB Live 5.1 sound card. All three systems are working well now (with the exception of the SATA in my main machine
).
Thanks for all your help guys. In the end, it was a video card that went bad on me unknowingly....