4 GB's of RAM
Jeff Atwood posted a great write-up detailing some of the issues around getting a PC running with more than 3GB of RAM. This is a great read if you're looking at building any kind of high-end workstation or if you're considering a 64-bit OS to support it.
I ran into the chipset issue he describes on my x64 system at work. It was confounding understand at the time.