@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.
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.
Either the Radeon R9 270X or the NVIDIA GTX 760 (which is basically a re-branded GTX 670). I'd say wait a while until the next GPU's arrive. GTX 660ti is fine for now. If you wanted to get max settings at 1440p you would need to spend a good $300+.