Forza 3 has a 2009 release date with 400 cars and 100 tracks.

[Dr.KennethNoisewater]

400 cars - not that impressive. 100 tracks is nice though.

Accoring to Wiki:

GT1 - 178 Cars/11 Tracks
GT2 - 650 Cars/27 Tracks (Already beats Forza 3)
GT3 - 150 Cars (Its back down, but the cars contain much more detail. Not sure about tracks)
GT4 - 700 Cars
GT5P - 71 Cars
GT5 - I predict over 1,000 cars.