A safe estimate would be go with a mid range CPU and a good video card and a decent ammount of memory.
1.6 Ghz Athlon XP or equiv.
256 mb DDR 266 or 333
Geforce4 (don't get the MX, and spend as much as you can on this part)
WinXP or Win2k Pro
That should be acceptable for any game right now and for the next 12 months or so.
I have an athlon 650 classic, 384 mb PC100 RAM and a Geforce3 and I can run almost any game right now without too much annyoance.