Well if you build it yourself than a Micro-ATX will fit under a seat comfortably. As well as a nano-itx, or Flex-ATX. If it is going in the trunk, than who cares about size?
If you don't want to build it yourself, you can get the Ampie case fully built with M1-ATX, M10k, and RAM for real cheap from the store. Of course, if you do it yourself you'll save.
Any reason you are looking at full systems and not just components?