I'm building a CarPC too
My setup involves:
1 x 7" Lilliput Touch Screen (motorized)
1 x Bluetooth Dongle
1 x GPS Dongle
1 x Laptop (NEC)
Video: USB Cable from the touch screen to laptop... works all functions just as the mouse would, just utilizes touch screen.
Sound: Optical output from laptop going into the input of my Pioneer head unit. Optical cable will provide sound that is second to none provided bitrate of MP3s are good.
Bluetooth: To be integrated with my mobile software ... instant car kit for phone, displays SMS on the screen (100% legal)
GPS: To be used with "free" GPS Software that I "found"
Why use laptop:
My laptop is provided to me by my work... it is the very very small (about the size of an A4 piece of paper and 3cm thick). Contains 120gb hard drive, DVD Rom, USB, Optical Sound Out, Video Out, etc etc.
Using a laptop also for security; don't feel like leaving big $$$ in the car as I've had a break in before.
Also, it'll be a lot easier to load new things onto it with a Laptop over a true In-Car PC.