You could use a micro-ATX mobo, and build a suitcase computer. Some micro atx boards have AGP ports and everything. You could put an LCD screen in the top part of the suitcase. That was you would have a nice computer, upgradable and portable. the only problem is, you wont have a battery, so airplane/bus/car use is impossible.
the highest voltage in a computer is 12v, so i guess you could rig up a car adapter for it. just clip the atx connector off of a dead PSU, or buy one, then hook the correct voltages into the proper spots. I wonder if that would work?