We have 3 PC's here with Doom3.
Mine is a P4 2.5ghz, 1GB RAM and a FX5600 256MB video card - plays comfortably at high quality 1024x768.
The next one is a Dual PIII 850mhz, 1GB RAM and Geforce 512MB 6800 video card - plays comfortable at high quality 800x600 (video card makes up for CPU)
The last one is a P4 3ghz, 1GB RAM, Geforce 6800 256MB video (wifes system - occasionaly used for Everquest..
) - and it plays perfect at 800x600 ultra or 1024x768 high.