No, I guess there isn't a real reason that I can't upgrade my case. I don't need a super fast machine, this one would be mostly a server/workstation type setup. If I were to upgrade to an ATX case, I would need new RAM, case, and power supply. If I just upgrade the mobo, I can still use my old power supply, and RAM that I have lying around. Plus, the case I have is a fairly nice one, with lots of expansion slots. A similiar ATX case of decent quality would be $50-$100, plus the cost of the power supply.
The mobo in question is a Tyan Tsunami S1830. I do need the ISA slots on it, which is one reason not to upgrade to a ATX board.