For awhile I played around with having a computer in my car, but gave up after not getting decent sound and my screen sucked. I was using a small fm broadcaster. If I were to get a new stereo I would wire it directly to it. Here are some useful links that i have used:
http://thisstrife.com/carproject/index_chapter_4.asp
http://mini-itx.com
If you want to buy that really nice touchscreen monitor directron sells it:
http://store.yahoo.com/directron/700tsv.html