From an interview with the NFS:Underground team:
"...We're taking great care to make sure that everything that should be customizable is. THere will be over 20 areas that can be visually customized, from paint and vinyl to installing neon on the car. If you can change it out on a real tuner car then you'll be able to swap it out in Underground...
Game Informer: How many different combinations of vehicles are there?
NFS:Underground team: There will be 20 fully licensed stock vehicles in the game that the player can then customize. All of the coolest cars that kids are tuning today are in the game, from manufacturers: Toyota, Nissan Mitsubishi, Subaru, Dodge, Ford, Hyundai, and more"