I'm a newb, but have done a ton of reading so far.
here's my comments.
a lot of people complain about the radio shark. but you already ordered it so /shrug.
any PC you can build today will be powerful enough to play divx movies/dvds and emulate any of those systems.
how do you plan on getting the sound from your cars PC to the speakers?
How big of a screen do you plan on using.
what size radio are you replacing? din size.
if you have 5" in height or so to play with, the xenarc 7" touchscreen seems to be a very nice screen. Power is already regulated and if you have a camera or console hooked up to it, the screen will auto switch to that video source once power is on that video input. It bypasses the PC altogether.