SFF Gaming Rig Help!

Rammy

Honorable
Both Thor and Affroman are right really. There are mATX boards that might meet your needs, but the selection is going to be pretty limited. The biggest issue is finding a suitable chipset as the majority of mATX AM3+ boards are at the low end which means slower ram speeds etc. The only board I can find which meets your needs is the Asus
M5A78L-M/USB3 which is also based on the 760G chipset.

As Affroman pointed out, with Intel you can meet these requirements fairly easily on ITX, and the mATX niche is very well covered.

It's perhaps worth reconsidering your priority, and either opting for a larger case and ATX form factor, or swapping to Intel (if it isn't cost prohibitive) for a better selection. As a side note, you didn't mention which CPU you have/are considering, and some motherboards may not support all AM3+ processors despite the common socket.