Mine's a little old (2+ years), I'm thinking about upgrading since it chunks running the newer games.

Athlon 2400+
Radeon 9700 with 256 Mbytes
512 Mbytes RAM
19" Sony Trinitron monitor and 17" Viewsonic flatscreen (two-headed monster)

A new AMD 64-bit machine (maybe dual core) with the almost-latest vid card is in my future, and I'll probably get my girl a laptop to a) keep her off my machine and b) allow me to play games while we're traveling.