Looks like a custom form factor to me
http://www.computer-reclaim.co.uk/dell-optiplex-gx100-socket-370-motherboard-85-p.asp
Thanks P1n3apqlExpr3ss,
I have an OptiPlex GX100. I am happy with it but it is too slow. It only supports 512 MB of memory, which I have. I would like to find a way to increase the memory.