Get Low!
I have this game for both PC and XBOX, the PC version you can crank up the graphics and AA/AF settings so it looks ultra sexy, the XBOX graphics are decent. The gameplay and unlockables are what make you come back for more in this game.
You start of with cheap low end import car's that are selectable. You race your way to the top of import racer scene and upgrade your base car and associated upgrades along the way. You can really tweak out your ride, the best part is if you own one of the vehicles in the game, you get to see how upgrades look to your car before you actually do it in real life! Cool.
Like Lil' Jon says, "Get Low".