@insane_shadowblade85 So this is not an accurate indication of how much better your games will run, but you can search for 3dmark results of different PC setups to see how the score will change when you upgrade a part. Don't look at these scores as absolutes since the overclocking varies heavily between these setups and some of these scores might have been run with overclocking that might crash the pc anytime.
Your old PC with FX-8120 3.4ghz and 7850: 4627
FX-8120 3.4ghz and GTX 970: 7466
FX-8120 3.4ghz and R9 390: 7489
Overclocked FX-8120 4.4ghz and overclocked R9 390: 8779
Overclocked FX-8120 4.4ghz and overclocked GTX 970: 8850
And if you upgrade your CPU to FX-8350 which uses the same AM3+ socket:
FX-8350 4.8ghz and R9 390: 9996
FX-8350 4.8ghz and GTX 970: 10082
And if you only upgrade your CPU and keep the old GPU:
FX-8350 4.3ghz and 7850: 5141
My advice is to upgrade your GPU to R9 390 or GTX 970. If those are too expensive then R9 380 is next in line and under it is the GTX 960.
This. Upgrade the GPU to one of the following. The rest of your system is good for another few years. And yes you can choose between AMD or nvidia GPU as they both use the same connection bus (PCI) unlike CPU's which each have their own socket type which in turn requires a specific motherboard for most.
I built a new system recently and I can tell you a GTX 970 is more than enough for 1080p gaming.