I recently got the missing pieces to get my car pc working. I have all the necessary parts to get it working with the basics, although I will still need a touchscreen lcd to make it better.
I already have a gentoo system booting off a 4GB usb stick. Pretty much everything I'll need is already installed there. The board for it is an Intel 945GCLF2 and it has 1 GB of RAM. It boots in approximately 20 seconds, and I can still shave off a lot of seconds from that by disabling unnecessary services.
To get it in the car, I'll need to move the board to the M350 that I recently got. I'll also need to put in the M3-ATX power supply from mini-box.
In addition, a gps device and an obd cable are going to be added to the system to log data. I'm also planning to get a 3G based connection and have it shared wirelessly.
update 1
todo: find a good 2.5" hd and install it in the system
install xbmc and use it as the primary gui
-- autologin and autoload xbmc on startup