I dropped by the Company Store a few days ago and picked up a copy of Forza Motorsport and a HD cable for the Xbox. Let me tell you, this is the most incredible racing game I have ever seen. Some of my favorite features:
1) As you replace components of your car, the engine and exhaust note change in a way that is really cool
2) The tracks are so detailed: in the New York track, you are actually driving through Times Square and you can point out buildings like the Virgin Megastore, etc.
3) You can adjust almost anything on your car. This really has a huge effect.
4) Replay mode: it is almost as fun to watch replays of your races as to actually race them!
If you like racing sims, run, don't walk, to pick up Forza Motorsport. You'll be glad you did.