actually im going to be integrating a computer into my car soon enough as well.
www.mp3car.com has everything youll need for research. I plan on not going touchscreen and molding a trackball into my center console. ive got an EMS so im going to be using the screen for programming on the fly and sensor output. Im also picking up a gps mouse off ebay for like 40$ and using my screen for a navigation system. you can get a cheap wireless 802.11g and connect to the internet at hotspots, play your mp3s off the hard drive, if you have a nice alarm like me you can plug into the alarm and program it change settings etc. get voice control of everything or write emails etc all through voice synthesis in a microphone. if you have a later s14 you can plug into the obdII as well. limits are endless and you can pull one off really cheap too.