Using a laptop has pros and cons. It's simple and compact and easy to run off DC power but you can't usually have it power on automatically. Assuming it has reasonable power management you can usually configure it to shut down or hibernate when power is cut, having a battery is a real plus.
I used a low power Epia motherboard with an Opus automotive power supply. The Opus is a DC power supply with a switching circuit that can turn the PC on and off automatically for you so it starts up when you start the car and shuts down after you switch off.
What head unit do you have? For me that was a big factor as there is no aux in on my CDR23 so it always sucked, 20Gb of MP3 files and only an FM transmitter to play them through.
GPS is awesome though, you can also add wireless internet pretty easily. I played with writing some apps to downlaod traffic data while driving.
The place you want to be is
www.mp3car.com you'll find all this and much, much more!
Chris.