Your parts are good.
What games you play will determine what is a good upgrade.
There is not a lot of improvement possible for a I5-4670K if you have overclocked it.
But, such games as strategy games, sims and mmo respond best to fast single thread capability.
A I5-6600K with an overclock is as good as you can do today.
If your games are multiplayer, then many threads is important and a I7- 6700K is optimal.
If your games are fast action shooters, then graphics is more important. GTX1070 would be appropriate for 1080P or 1440P gaming.
If you are looking at 4k gaming, then GTX1080.
How to tell?
Here is my stock approach to that:
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To help clarify your CPU/GPU options, run these two tests:
a) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.
You should also experiment with removing one core. You can do this in the windows msconfig boot advanced options option.
You will need to reboot for the change to take effect. Set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system,
and both cpu and gpu need to be upgraded to get better gaming FPS.
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