I like Forza Motorsport. It's more realistic than GT4. Graphics wise GT4 is better but there's just more you can do with Forza. There's serveral vipers in the game.
ACR
GTS/R
SRT/10
Competion Coupe concept
Paul Mumford's CC
Tommy Archer's CC
I got NFS: Most Wanted and already unlocked the SRT coupe. It's a great game, it's very challenging. There are 15 blacklist rivals. It takes on the role of Gone in 60 seconds but instead of stealing cars your working you way up the list. It has a very cool atmosphere. When you beat a blacklist rival you get 6 markers to choose from. You can only select two of the six but if your lucky you can get the pink slip to their car.
I've got 50% done with the game. I owned a Cobalt SS and won a Supra, I drove the Supra for a bit and won a Eclipse. I kept all three, then sold the Cobalt SS for some extra money to get a Aston Martin DB9 and some extra upgrades. I keep the Aston for 3 races, won a Lamborghini Gallardo and bought the viper. I've got 4 blacklist members left.