Since you noted Doom3, the bleeding edge video card is what you want. I didn't see the CPU to be the bottleneck (any P4 2+Ghz seems ok), but the video card and 512 or even 1G of RAM seem to be killer items if you want to push the game to the edge.
Or you can do what I do and run the game in reduced modes.
which is better?????
and what config is MINIMUM req. on each to play games like Doom3,Far Cry and half-Life 2 etc...