If you have an Xbox get Forza Motorsport. I played it around a friend's house and it was fantastic. Full damage, you actually need to break for a corner etc. Much better than GT4.
In terms of car game, depends on what you want. If you want an arcade thing go for Trackmania:Sunrise. Awesome little time waster. Simulation wise i've heard, as MF has said, that GTR is the most realistic simulation around.