I never fought Forza 3 would be able to be such a huge upgrade graphically over Forza 2. I mean, Forza 2 wasn't even a launch game, far from it.
The demo of the Forza 3 photo mode (it's confirmed as replay/photo mode, rite?) where the cars switch places was the best in-game graphics I ever witnessed - if they're true.
(although some would argue that calling it 'in-game' is a stretch, since it's replay/photo mode)