nwo is right. If you have a 6800 ultra in one machine and am fx5700 NU in another identical machine you will be able to play doom 3 on high detail at a good 50-60 fps. The fx5700, however, will give you a slideshow framerate around 5 and will not be playable. I'd say that's more than an undetectable difference.
it's like saying an athlon64 and p4 clocked at 2ghz run the same. they most definitely do not.
btw i'd also recommend getting a 6800. The other cards just aren't worth it anymore due to the huge increase in performance with the current generation