The system I have sat on me worktop at the moment and ready to install is:
*Mini ITX Case - head unit sized so will slot straight in place of my head unit.
*1 GHZ processor - is biggest can get in such a small case
*512mb RAM
*100GB hard drive
*CDR-W / DVD drive
*External Sound card with 6 RCA outputs to feed amps
*7" Touchscreen
*USB 802.11 Wireless LAN device - so can connect to home network
*Bluetooth device for connection through Mobile phone
*Sat Nav
*Camera control unit for up to 4 cameras Wireless @ 2.4ghz
*2 camera infrared - 1 for front view 1 for rear view - in car for driving vid's etc
Running Windows XP with software for Sat Nav, DVD's, VCDs, Mp3's, Camera recording etc etc etc.
There's other bits and bobs too, just gotta find time to get in installed into me car.
I think In Car Computers are the way to go, you can do soo much with them and they dont cost the earth neither.
Hornet