Forza 2 is a great racing game, if you only like racing on closed tracks with little scenery. The demo was shit, but the game is great.
PGR3 was fun, PGR4 was better. You can have fun with both games, I enjoyed PGR4 just because of the drifting, kudos and in-car camera aspects.
Dirt is fun, can't really be compared to the other two.