you sure it's the motherboard and not the power supply? I could see the PS getting goofy too.
Regarding upgrading the box, good luck. With a machine that old, you'll probably have to buy pretty much everything new. Still, you can go with the cheapest parts you can find and still end up with a significantly better computer.
Personally, I have enough parts lying around to build at least one or more computers.
Though I've started to get rid of stuff I'll never use.