Well,for AMD FX processors,you need an AM3+ motherboard.
You have a couple of options:
1. ASUS M5A97 LE R2.0
2. ASUS M5A99X EVO R2.0
3. ASUS M5A99FX PRO
4. ASUS Sabertooth 990FX and finally
5. ASUS Crosshair V Formula.
The Formula is the best,but also the most expensive.I would get the M5A99X EVO or the Sabertooth,but it's totally up to you.
AMD FX processors are the best for gaming.FX-8320 is by far the best option.FX-6300 is also an excellent choice.
But for next-gen gaming,an Intel Core i5 is a way better choice than AMD FX series.CPUs like i5 4670,4670K,4690 and 4690K will outperform the FX 8320 in some games,while the 8320 will beat an i5 4690 in some games,in terms of FPS.
All the best.