Expect my full review next weekend... I've been playing Pro Race Driver 2 for the last half a year or so to get my competitive gaming racing fix... The X-Box has some fantastic racing games (not driving, per se - GT4 does pretty well for that), and I fully expect Forza to be a fantastic addition.
Pro Race Driver 2 is about $14 at Gamestop when you get your Xbox, BTW... and it comes with Collin McRae Rally.