The answer is not clear.
Your i5-4670K is still a very good gaming processor.
GTX970 is still a good card.
Since a big gain from a processor swap is not easy to come by, I think a GTX970 to a GTX1080 class card is likely best.
If you decide on a small cpu upgrade, you can drop in a I7-4790K and OC it.
Your clocks will not be different, but you will get 4 extra threads which will be good primarily for multiplayer games.
Past that, you should be looking at a 8th gen K suffix processor which will OC to the 5.0 range.
You pick the number of threads, 4/6/12.
That will entail a new z370 motherboard and ddr4 ram.
What is your budget?
Some games are graphics limited like fast action shooters.
Others are cpu core speed limited like strategy...