[JiveTurkey]

I think you're wastin your time but for your question I've seen a couple small form factored boards. They were a real bitch to work on in a tiny computer case and had no upgrade ability. I've heard them called Baby - like BabyAT or BabyATX. Scour usual comp hardware sources or eBay for some poor bastard stuck with some.

You haven't really stated what you're attempting with it but if you're willing to change the board why keep the Xbox case? In theory if you used a comp mobo then you could put it in a comp case and transfer all the Xbox components into the comp setup. A computer with Xbox guts would be um L33T. Just an interesting idea.