Nice concept! I'm thinking about building another CarPC as well. The last project worked nicely, but died before I ever had any chance of putting it in a car. The draw back was the size of the box. It was a hand-built acrylic box with mini-ITX board, modified X-Fi soundcard, and 2 CF card slots: one for OS and app, and the other for music storage.
Anyhow, I've been looking at building one again. For audio out, I may go with USB audio. Check out
Home | miniDSP
They offer small DSP boards for audio processing (cost of plug-in apps are $10 each with lifetime updates). Depending on the combination of boards, you can get it to accept analog in, SPDIF, or USB. Output can be digital and/or analog.
So I was thinking about building a small computer, use the USB as audio, and the MINI DSP board will connect directly to my amps. Not only will I get clean audio, the MINI DSP has advanced audio processing capabilities (EQ, delay, xover, etc.).
Whatever it is, I'd like to be able to get access to my music and switch songs faster than the current DDX-814 head unit.