Loadup of your OS has more to do with your hard drive speed. Having a powerful computer in the car to just play music is overkill, IMO. If you're gonna spend bank, might as well get a Core Duo or Turion setup, which draw very little power (<35 watts) and are pretty speedy. That's how I would do it if I was to start from scratch, although my 50% underclocked Barton is plenty.
Those little Dell microATX cases are pretty spiffy though.