I have a completed carputer system for sale with the following specs. The system currently has XP and media car installed and is tweaked (boots and ready to go in 15 seconds after post)

Small form factor Desktop PC
(Floppy, CD and one HD slot only w/ 90w power supply)
Intel 810e Chipset
Pentium III/733Mhz
256 MB RAM
80 GB HD (less than 3 months old, no bad sectorrs)
On-board video and sound

$300 plus shipping (o.b.o.)

I have since abandoned this system and opted to go with my spare laptop, so it is up for sale. The system worked great on the bench and performed far better than anticipated.