This one I'd say the first thing is the GPU. The 770 is great and it holds up real well, but you will see a huge increase if you get a 1060 or 1070. The 1070 is sadly above $400 right now, but the 1060 can be found for as low as 260-270. You will absolutely see an improvement in games with that.
After that, I think CPU should be next, but the thing about the CPU upgrade is that you will most likely need a new mobo as well, I dunno that's just my two cents.