You might like 'Grid'. It's an arcade style racing game, but it does have plenty of realism in it. I find the AI to be pretty challenging. I have this one on my PS3, and it has different driving disciplines such as grand prix, drift and street racing.
http://www.gamespot.com/xbox360/driv...ml?sid=6192103
Another game I like is DiRT. This game is also by the people who made 'Grid'. DiRT is a rally racer with arcade and sim elements. I love it. Great graphics, good AI, plenty of variety. The multiplayer doesn't have much to it. That's the only drawback.
http://www.gamespot.com/xbox360/driv...largames;img;1
There is a Dirt 2 slated to come out this year.
---Enjoy, Jason