The Phobos Lab has got the official minimum system requirements for Doom III from John Carmack.
|With all the hubbub floating around the net lately about what would and what wouldn't run DOOM³, we decided to bring an end to the system requirement confusion and contact the main man, John Carmack, himself. Here is the official word on the minimum system configuration you'll need to run Doom³.
I guess this are the minimum requirements to play the game at 640*480 with low detail on.. But anyway I've heard that some people were able to get an average of about 5FPS with a Pentium III 500MHz, 128MB Ram and a GeForce 4 Ti 4600 in the unoptimized and illigal alpha version of Doom III which leaked in November 2002. So maybe if you have a good 3D Card, and 256MB Ram you will still be able to get decent performance on a 1GHz AMD or Intel CPU.