I'd say comparing PGR1 to PGR2 shows a huge improvement. Same goes for Splinter Cell 1 to Chaos Theory (an arguably even bigger jump). I also very much disagree with those that say that God of War 2 looks better than anything on Xbox. I played God of War 1 for ~ 20 hours and I saw it as the 3rd game (MGS2 & 3 being the others) that could be mistaken for a game that came out on the Xbox, though even in that case there were things that made it a very PS2'ish game (mostly lack of shader-based effects, specular, normal mapping). MGS2 was, at the time the best looking game I had ever seen, and I don't think that anything significantly surpassed it, with the possible exception of MGS3, though even then, the combination of an unsteady 30fps with 3 (? i think?) years of time passing didn't impress me nearly as much (I also think that it tried to do too much, and I liked the new-age industrial environment more in the first). But I digress.
I really believe it depends on which devs you look at. Even in the PS2's case, there are games that came out in year 2 (MGS2) that looked better than 95% of all games to come. And the PS2's case was special because at launch it had almost no dev tools, and most games lacked mipmapping, and used a half-height framebuffer. So without changing anything else, just improving on those 2 gave games a huge jump (GOW 1 I remember looked very clean because of that).
