You have a BTX motherboard which is "backward" from the ATX standard most PC products are made for... unfortunately that also includes cases which makes finding a nice aftermarket case rather difficult*. Be wary of claims of BTX compatible cases that do not come from Dell, HP, Gateway or AOpen, some manufacturers claim BTX compatibility but they are not.
You're going to be looking for a BTX PC case ... Good luck and let us know if you actually find one
*read: impossible