Ive searched a little and have only found people using powerstreams 90watt and 70watt units. But they have 250-500 Watt power suplies for the car. 9-18volt input.
Here is the link
http://powerstream.com/DC-PC-12V.htm Does/has/would anyone use this power supply?
Would it work for a gameing carputer? I could build or buy a startup/shutdown controller if need be. I just wanted something to power a P4 3.2Ghz system with a gig of ram and a 128meg video card

Maybe 500 is too much. But why go lower. This way I could use any motherboard cpu combo with no worries on power.