My question is... How the hell do I figure out what motherboard I have, so that I can be sure I order the right ram?
Was it purchased, or built by hand?
If it was purchased it should be pretty easy to look up the specifications of the computer on the 'Net, then take the memory type from that list to a known configurator like Newegg or Tigerdirect.
If it was built by hand you'll have to open the PC itself and inspect the motherboard directly to find its model number.