anyone with a 900 mhz processor, 512 RAM, and a decent 64mb video card, can run Doom 3 at low quality.
I'm not doubting that, but I have a 1.4 Ghz processor, 512 PC2100 DDR RAM, and a 64 MB Geforce 2 MX video card, and I can run DOOM 3 at 15 FPS at most with all of the lowest settings enabled. My average framerate is 8 FPS. The biggest problem for me with DOOM isn't really the number of monsters the game throws at me, but rather the complexity of the maps in combination to all the particle effects that are generated when a fireball is thrown. Perhaps there are people more tolerant than I, but anything below 5 FPS becomes near physically impossible to play.