Your total solution is called M2-ATX, I've been using it now for a few months and it is wonderful!!!, I mean I have no ways of telling you how much I like this piece of hardware!!!, Its definitely what makes a carputer a carputer!!!,
go for it, because it will take care of all you power concern issues as it has preprogrammed shutdown schemes, for me is the best for the money, I paid like $80 on ebay, I guarantee you will love it...
Of course there is the competition that has programmable serial interface but it is more expensive I dont remember the name now...
Good Luck...
PS. always remember to have adequate cooling so you extend the life of it....
