Screen : Many people seem to have good experiences with Lilliput (a good value screen). VGA screen for better quality, at higher cost (look for dscustoms).
Power Supply : Opus (or conventional ATX PSU + inverter for the cheap solution).
Sound Card : Extigy (or whatever you like).
Video Card : Something with TV out to drive the screen in you have a composite screen.
Mobo : If you want a smaller mobo, go get one, if you don't, then don't. The choice is yours. Personally, I got a small one (well it was small when I got it over 4 years ago).
Build the system, hook the power up to your battery. Hook the ignition sense from the Opus startup controller to an iginition line. Hook the TV out from you graphics card to the screen. Power from the power supply to your screen, and lineout from your sound card to the aux-in on your headunit or amp.
Install your software, load your tunes, figure out a control method (keyboard, touch screen, IR remote, etc), and off you go.