Your current pc is very capable.
I do not see a magical update that will give you a fps boost.
It may well be determined by how cpu intensive YOUR games are.
Sims, strategy and mmo types tend to be cpu limited and need few cores.
Fast action shooters may be more graphics limited.
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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. 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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Your HX750 is sufficient for a graphics upgrade either to a GTX980ti or to a second GTX980.
If you are cpu limited, a i5-6600K with a good oc can give you a nice cpu boost.
The hyperthreads of a 6700K will not be very useful, but the 6700K is likely to be binned better and give you a higher oc.
FWIW, Skylake 14nm runs cool, obviating the need for liquid cooling today.
Next year when smaller graphics chips are launched, we should see less need for liquid cooling of graphics cards.