I've been playign with SBCs for quite some time now. Here's the deal.
There are 3 main types
PICMG- requires backplane usually, also the largest size (full size). easiest to add a PCi card to as it uses a backplane
Half size PCI/ISA- usually takes a 4 pin molex or Aux 6 pin aux or even a full ATX. Can be attached to passive backplane or riser to add more PCI cards. MUCH smaller (half the size) of a PICMG
Biscuit- same as the PCI, usually in 3.5 or 5.25 sizes. Sometimes has a PCI slot built in.
For carputing, i would suggest a s370 model. Go either celeron or p3 at 800-1.4ghz, if you can support it. Pay close attention to the bus speed and everything as you want to pick the best processor for your board. Also, pay attention to whether it takes Sodimms or SDRAM. Sodimms are smaller and take less power, but are more expensive. The only good news is that you can more readily resell them for older laptop users and at a premium due to their rarity.
I do have a friend who had one for sale, a full size with backplane, and i had one for sale, w/o backplane but i could get you in contact with my seller.
SBCs are easy to set up- just read the directions