I built just about the entire thing out of spare pieces I had lying around.
The computer:
1GHz Pentium III processor
384MB RAM
20GB hard drive (temporary)
64MB AGP video card
Aureal Vortex 4 channel sound card
TM-868 off-brand 8" touch screen
Sound:
MB Quart 6.5" components in front
Kenwood 320W 4-channel amp
Obcon 10" subwoofer in trunk
Jensen 300W 2-channel amp, bridged
Pictures
The car (slightly outdated):

The computer is a full-size ATX mounted in a custom wood case (again, I used parts I had lying around, this is not a perfect design by any means):

It sits in the corner of the trunk:

Power to the computer and inverter is controlled by switches in the dash until I can finish a startup/shutdown controller:

To mount the screen, I molded the casing from the screen into the stock radio bezel using epoxy putty and bondo:


And here it is mounted in the car:

Right now I'm using Roadrunner as a front end because I like to be able to customize it and it seems pretty stable, although I've been having annoying problems with the computer a lot so far (in the last two days, I guess it's not that bad

There's a ton more to come, but it's a slow process because I'm poor.