GRID, Burnout Paradise, and Midnight Club LA are my suggestions.
Start with Burnout Paradise. It's got an absolute shitload of content, it's polished, the visuals are nice, it's fast, it controls well, etc etc it's a cool game that should be supported by cool people. Oh and its only twenty bucks, which is really great considering how good it looks and just how big the game is. You can buy it off of the Playstation Network or at retail.