You most likely have the BAT case (it says "BAT" on the back). This is actually a 3/4 AT motherboard case. Most AT motherboards are small enough to fit. I have some motherboards with the 2 PS/2 connectors if you're interested, they'll officially support up to 200MHz, but since 1.5x=3.5x, you can get them to 233. A better choice would be something newer. Pine is a company that speciallizes in AT motherboards for new processors (PIII, Celeron, Duron, T-Bird). Tigerdirect and Global both cary their products. Personally, I would suggest you go with a new ATX case, or buy one of my old Motherboard/Processor combo's to upgrade the PC (make it sellable), then sell it.
One last option is to get a super 7 board, but since these cost as much as Duron/Athlon motherboards, and matching processors cost as much as Durons, you might as well get a Duron system together.
What's the frequency, Kenneth?