I'm not up on current AMD platforms, but I can tell you that you are better off dumping the FX-8120 and getting the FX-8320 or FX-8350. There is only about 30 bucks difference and the 8320/8350's perform much better than last generations 8100 series.
Save some money and go with the Corsair Vengeance 8gb 1600MHz RAM(2 x 4gb sticks). You need an even number of RAM sticks, and as cheap as RAM is, you either need 8 or 16gb. For gaming, you'll never max out your 8gb.
Spend the money on a good SSD for your OS drive as it is the component you will see the greatest improvement in. Nothing else will give you the performance boost an SSD does.
Lastly, we have to do better with the video card. A video card at 100 bucks isn't going to do much for you....you might as well get an APU, which is a CPU/GPU in one package. For a "gaming" computer, there is no reason why you shouldn't spend as much on a graphics card as you did on the CPU, in my opinion. I would look into the Radeon 7850 series to start and if you want a card that will last a little longer, you can go with a 7870GHz edition.
I don't know what your budget is, but I'd rather spend the money on good quality, viable parts, than spend a little less and get parts that don't perform well, and have to do this again a year or two later. Buy once, cry once. If you have anymore questions, feel free to ask.........I'm sure an AMD expert will weigh in shortly and can give you some more advice.