Instead of giving you concrete answers (which there aren't any in this 'car-puter' case, 'cause no 2 setups are the same) you should do your own research.
First Determine what you want the computer to do: Playing mp3's/DVD's won't be too dificult. The same goes for GPS..want to put a pc in that will play mp3s,dvds, have GPS, be able to control my Eclipse 6 disc in dash changer, amps, etc., be able to control my a/c/heater controls, monitor gas mileage, and maybe hook a PS1 up in the future. Also I would like to have some reminder of when to do maint. like oil changes.
You will find a lot of trouble when you want to control the CD-Changer, and also with the A/C-Controls
Do some research yourself in this BB or you might check out
Examples Pages from MP3car.com
If you get a little deeper into this, you will find out that not a single setup is the same as the setup from someone else. Therefore it's not possible to give you an "out-of-the-box" solution.
I'm sure you will be getting enough Idea's by just looking at the sample pages. e.g. You read something, and you think it might apply to you, Write it down or something.
I'm sure when you read a lot of this stuff, the 'total' picture of YOUR Carputer will become much clearer.
Hope this helps you out !