Are you gonna hook it up so that it will start up the OS once you turn on the ignition and then shuts down when you turn off the ignition?
I used to have something like it, it's called the M1-ATX. But now they have the M2-ATX power supply. Check them out in ebay if you don't have them yet.
Sample:
http://cgi.ebay.com/M2-ATX-DC-DC-ATX...3A1%7C294%3A50