Actualy from the way I understand it, its on par with the gf4 4600.
One thing I don't get though...In carmacks press release he did say the gfx would be every bit as good as the PC, but I can't see how thats possible given the limited memory and processor speed of a console for a game thats said to need a 128mb graphics card, and at least a ghz range CPU. Hell I guess since its a fixed platform, they could make system specific optimizations? *shrug*
I see why they picked xbox though, probably not a whole lot of porting to do, as well as it being the most advanced console there is right now.