Just a few thoughts on this, I would probably stay away from the amd processors for a car computer, they tend to run quite a bit hotter than the intel processors. If you look around you can find some pretty cheap parts. I got a foxconn mobo(they just started selling stuff under their own name) for about $70 with onboard everything. I picked up a celeron 2.4 for it for $75 or so, if I remember right the 1.8 was about $50, but for that little money I figured some processing power would be nice. I got my stuff from www.pcclub.com, they seem to have a pretty good selection of microatx parts.
You'll probably want to go with socket a/478. 478 is the common p4/celeron. socket a is the general amd chips