PGR4 is you haven't played it before. It's great for just jump-in-and-drive yet reasonably realistic physics. Great online races. And a good single player mode if you need to get in some practice. Nothing beats the feeling of driving against other humans though.
Haven't tried Dirt 2 but didn't like the original.
Just been playing Forza 3 for the last few days. Excellent game, if a little bit tweaky for my liking. Single player mode is a little bit boring, but man, 400 cars and 100 tracks?? Can't go wrong. Circuit racing in A-class cars (high end Ferraris, Lambos, Audis etc) seems to be the most popular online component – and the physics are ultra realistic – try taking the assists off and you'll be off the track in the first bend!