Forza is a very good game, but it's also a lot more realistic (read: harder) than Need For Speed and the number of tracks is rather limited.
Grid could be good - download the demo and see how you like it. Otherwise, the Project Gotham Racing series is super cheap these days. PGR3 is only like $5-$10 used and PGR4 is only about $20, so you may as well get 4. The physics are exaggerated to make it easier without ruining the fun (in fact, they sort of make it more fun), the cars all handle differently, and there are a zillion different tracks since they're all created from various road segments from a bunch of different cities, just like in Need For Speed.
I've also heard DiRT is good, if you're into off-road rally racing.