Well GT3 wouldn't exactlly push an GC or Xbox GPU at the moment, the great thing about GT3 is it uses lots of visual tricks to make it look great without much rendering over head.
What I mean is take a look at Project gotham it does at least double the polygon counts and has 7 realtime reflections running at once (6 for cars, 1 for the road, it's active in both rain and not rain tracks). Now PS2 would have no hope of replicating that kind of rendering but it still doesn't make PGR look better than gT3 now does it.