i know of somebody who did this to their car. let me find you the link, and it'll have LOADS of information of as to how he did the install
EDIT:
this is the one i wanted to show you. click on "the car" on the left side, then click on "carputer" after that on the right side. The forum link is
rx8club.com (go figure

), and the first post has the hardware specs he used for the comp. not to shabby for being what it is.
here is the secod one. not as detailed as the first, but just to give you more ideas.
i know that both cars are RX-8s, but you can use some of those ideas into your own car.