Well theres the 6800ULTRA and variations on it I think.
If you can SLI some 6800s then that might be the fastest.
The higher you go the less cost effective it is though.
Really depends on what you are going to be utilizing the car
for...gaming, editiing, movies etc.. I believe the Nvidia 6800 ultr
is the fastest however it is a little pricy at around $400.0