@adolescent
Even so, assuming that a 1GHz G4 fully altivec optimised version could only calculate half the polygons (and frankly I'd expect an optimised version could do better than that), youd still get up to 30fps if those shots are any guide.
Don't forget that doom3 is comparively low polygon - almost all of the detail is in the bumpmapping. Stripped of all of that and you have a managable level of detail.