I'd go with FX 8, it competes with the i5 in gaming. FX 6 is more like vs i3. 8 series OCs great too, and, though it'll not be a significant difference in FPS, 5-10 FPS is a considerable amount. Moreover, BF4 MP is very CPU intensive and the 2 extra cores will definately help FX 8 there:
http://www.anandtech.com/show/6396/the-vishera-review-amd-fx8350-fx8320-fx6300-and-fx4300-tested/5
Also, FX 8 is definately more secure purchase for the future as game developers are likely to become more sensitive to multi-core soon. But with either processor, make sure to get 9xx series MoBo.
Also, FX 6350 is a great CPU too, but for $20 you're getting 2 extra cores, which'll come handy soon IMO. A great thread:
http://www.tomshardware.com/answers/id-1866711/amd-6350-8320.html