For arcade racing go NFS hot pursuit or wait a week for motorstorm apocalypse. Depending wether you like on-road or off-road racing.
For simulation get GT5. Single player doesn't give you much in terms of racing unless you make sure your car is at the same level as the AI. The special challenges were fun but a-spec was mostly a grind until the extreme level. Fun for the driving, but not a lot of interaction with other cars going on.
Online racing however is great and has me hooked since December. There are so many race types, from bump drafting carts on Indy to racing nascar on le Sarthe to taking le mans cars to the streets of Rome. Never gets boring. Driving a tuned street car without any assists on Nurburgring with variable weather and accelerated day/night cycle, simply awesome. Got to go race now.
