Yep, the APU should disabled once you install the GPU. And if it doesn't you can always go into BIOS to disable it.
I don't know what sockets are compatible with the mobo, but the best socket FM2 CPU is the Athlon 760k
It won't run newer games too well, but low-medium settings shouldn't be a problem. Dual cores aren't good for gaming anymore so that CPU will hold you back in newer games for sure.