I'm using an old XP1500+ cpu n stuff that I had in that box.
Will be rubber mounting the motherboard and making my own box to go in the passenger footwell.
Found a 180W DC-DC PSU, it's more of a regulator than a power supply.. takes in 6-18V and puts out a regulated 12V. Nice bit of kit, about the size of a video card. Also allows you soft-off the PC just by turning the car off and it won't reset the PC when going from ACC to START (like your stereo does). It was also around the $100 mark so not excessive.
I'm doing this purely for diagnostics as I run a remapped stock ecu and have a consult cable and Nissan Datascan, which means I don't need huge computing power and graphics power. Will probably hook it into my stereo further down the track.
Found a couple of indash monitors at Jaycar so will be checking them out when I get a chance in the next couple of weeks.