AMD FX CPUs are power hungry. I believe even the 6 core FX-6300 consumes 100w more power than Intel's Core i3. Therefore, if you want to go with AMD then you need a more powerful PSU. I would not even attempt to put a FX-8xxx CPU in a mATX case. Too much heat and you don't really need 8 cores. The motherboard you select only has a 760G chipset so you will not be able to upgrade to Steamroller which needs a 9xx series chipset. Games using only 1 or 2 cores will run better with the Core i3, but games that can take advantage of more than 2 cores will generally run better with FX CPU.
Crysis 3 is an extreme case where a Core i3 performs poorly. Any other games using the CryEngine 3 game engine will likely not set the CPU demands as high as Crysis 3. The higher the requirements the fewer copies the publisher will sell. Unless Crysis 3 is on your list, the Core i3 is still a viable CPU and you can upgrade to a Core i5 CPU later. If you don't want to bother with upgrades in the future, then just go with a FX-6xxx CPU.
You can check out CPU benchmarks in several games from the following website to help you determine which CPU to go for. Simply click, then scroll down to the
Gaming GPU/CPU Performance section for several games benchmarked. Then look for the CPU Performance benchmark in each game.
http://www.techspot.com/guides-gaming.shtml
ARMA 3 is the most recent review. Here's a link and the benchmark chart.
http://www.techspot.com/review/712-arma-3-benchmarks/page5.html