You have to be careful with CarPuters.... Size and noise become big issues.
You can go for small with a small DC-DC power supply for whatever form factor you're using. Or you can try to build a home computer with an DC/AC invertor, just to convert it back to DC again to supply the computer. It becomes VERY bulky and uses a lot of juice. Not just that, but you have Shutdown issues, or if you leave it running while you're gone... no battery left.
A small computer like one built for a car is the way to go, but pretty expensive. The Mini ITX NOT ATX is really an awesome way to go too. But also, the small size is expensive. However, you can adapt some of your home computer stuff to go with it better and lower the cost.
I think I'm going to go with the mini ITX route. It's going to have to wait though, I think I'm going to buy a FMIC setup from Austrailia. I can't decide whether I want NOS or FMIC.... Maybe I'll do carputer AND NOS and wait on the FMIC...