I have done a similar project before. From my experience a very good resource for information is Mp3Car. There are a lot of projects with pictures. People over there is very helpful on this matter.
There are many solutions to powering your computers I have used DC-DC power adapter that transforms 12v to ATX compatible power (M4-ATX). It take of engine cranks and any voltage variation. A good place to buy parts is Mini Box they have a car computing section.
When it is not necessary to make a decision, it is necessary not to make a decision.