Shuttle is famous for making small PC's. They have recently announced their new PC for your car which looks very similar to an amp for a high-end sound system.
The car PC should pack enough of a punch for the average driver. Look for an Intel Core Duo and 7.1 channel audio and DVI-out. One of the biggest issues with car PC's is that you generally have to get a special adapter to shut down the PC when the car is turned off. You won't have to worry about that with this little guy, it's already built-in.
I enjoy the thought of having a PC in my car, but I honestly can't think of a good reason for me to have anything other than my laptop in there. I'm sure there are plenty of audiophiles that would get plenty of use out of one of these though.
Shuttle Car PC: A Techier Way to Die [via gizmodo]