I just got to test out the IWill DVD266-R motherboard I got the other day. I haven't got a boot drive on it yet, but with two matching Pentium III Coppermine-T 1Ghz CPUs, it boots just fine! With 2GB of DDR, dual 1Ghz PIIIs and a ton of BIOS options, this could be quite a beast.
I'll probably thoroughly test it out, play with it for a while, figure out the best well-rounded configuration for it and then sell it eventually, as it is apparently worth a pretty penny (I also have the box and accessories).
This is the model before the one with Tualatin support sadly, but I think its enough of a beast on its own. I'm thinking Windows XP or 2000 would be a great OS to test it with.
I know what it'd be really good at, but some sick part of me wants to see it run something more modern like Minecraft. The hard part will be getting a GPU into it that is up to date enough for the task.