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

[Squilliam]

RED53DEVILS said:
GT5 has been rumored to have 96 tracks and 600 cars but this could certainly increase

Which is a rumour which makes sense as Polyphony digital are insane workaholics and GT4 had over 700 cars and they wouldn't want to go too much lower than that number.