If you get a carputer, get an Opus
www.opussolutions.co.uk
check out www.mp3car.com for in car questions FAQs on anything you could possible have a question to.
i completed mine a month or two back, im well chuffed with it. Mini itx 1ghz pc with 256 ddr 20 gb laptop hd running off an opus. Its neat, quiet and consumes less than 50 w or power (as opposed to around at least 100w depending on your pc)
If you running agp, then you'll need about 20-30w to be safe just for that, if your cpu is an AMD - around 50w for that - full atx mobo about 60w.
Seriously conisder an opus or dc-dc brick style psu.
www.linitx.co.uk - uk based itx company
www.itxwarehouse.co.uk - just bought my new case off of them, well cheap and great delivery.
www.ultim8pc.co.uk - good range, helpful lot, few reviews
Any questions feel free to give me a email
John