The BTX form factor offers great cooling potential and super quiet operation... This one was used as an HTPC, but it could be adapted to work as a carputer. It is fairly compact, attractive, quiet, and not overly power hungry...

The case... Cooler Master CM Media B260 mBTX Case with power supply.

MFG link:
A review:

Case alone sells for $70:

The motherboard... Intel D945GCZL mBTX Motherboard

MFG link:

Board sells for $90+:,__9440602

You get the case which is in great condition, the motherboard which is also perfect, and a 275W power supply. 275W may seem low to some, but it handled a dual core 3.2GHz processor, 2GB DDR2, a hard drive, and DVDRW drive just fine for me.

I do not have the BTX backplate for the rear of the case, so that area is open. Maybe eBay can help with that for a few $$$.

Only thing special between ATX and BTX is the CPU cooler, so you will need a BTX cooler!

Price - $75 + shipping! I ship FedEx Ground with insurance, please PM with ZIP for rate (It will be about as cheap as USPS Priority).