I'm also in the process of putting an EeePC in my truck! I just took the dash apart last night to see how much room was there. Unfortunately, not as much as I'd like.
So far I've put nLite Windows XP on it, removed any and all extra junk. and installed a test copy of Centrafuse 2.0. It runs pretty smooth. I'm running a super-mini bluetooth adapter, paired with my motorola phone (which works incredibly good I'd like to add!). I've got a list of things planned, some of which I've already ordered. First being a touchscreen overlay. I don't care about being able to remove the PC from the car and I'm on a budget, so it was the cheapest route to having a touchscreen. For volume control I'll be using the screen and steering wheel controls (using a CarPC CarJoy adapter, or FusionBrain).
Why are you using an external soundcard for audio, if you don't mind me asking? My audio output will be from 3.5mm to RCA's to an AUX Input, or my amps should I bypass the factory setup.
Other programs I've planned on is: GPS (Centrafuse 3.0, or maybe iNavigation), ODB-II Display using Centrafuse unless I find another software I like better. (Don't care about having on screen gauges, Raw numbers are fine by me!), and other than media I'll probably be a nerd and run some game emulators. Not that I'll play them much, but they are reasonably small, and who knows. Maybe it'll keep the girlfriend awake on long road trips. *laugh*