UT2K3 runs on my 1800 athlon xp, with gf2 mx400 64Mb at over 40fps at 800X600 medium all settigns, and it looks quite good for a couple year old budget card. The GPU on the Xbox is the NV2A, a revision of the NV20, basically, the xbox gpu is a gf3 ti200 with a slightly faster pixel shader, So frankly saying, doom3 will most likely run on 1 ghz system, with 128MB, prefferably 256 or more, swapping is my worst nightmare, so if you have one of the following video cards, most likely you will be able to run at 640X480 or 800X600 with medium settings at not too bad of speeds, and of course faster cards will have no prob running at higher settings:
geforce 3 ti200
geforce 3 standard
geforce 3 ti500
geforce 4 MX460(this is really pushing it, but has 8GB/s mem bandwith may come in handy)
Radeon 8500 LE
Radeon 8500
Radeon 9000 Pro
Geforce 4 ti4200
geforce 4 ti4400
rememer this is an estamate, but imo a fairly accurate one, these are now a days the fastest, of the budget cards,