Doom 3 Hardware specs...
HoustonChronicle.com - Computing has an article on the specs for Doom 3.
No OS requirements, but it's all about x86 hardware... hmm.
My $700 homebuilt box will work fine it seems. Maybe I could spend $100 or so for a new MB/CPU to go faster but I should be good to go.
That $700 matches the performance of a $2500 G5 by the way and the cost is probably lower today than when I did the measurement.