What is the pc used for?
What is your budget?
For gaming, it may well depend on the games you play.
Many games use only 2-3 threads and are highly dependent on the performance of the single master core.
sims, strategy and mmo type games are in this category.
Fast action shooters need good graphics.
Multiplayer games like many cores.
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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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I might wait on a R9-390X upgrade; Pascal is looking to announce in April, and I would be surprised if amd did not have a response.
If you do upgrade graphics now, protect yourself with buying a EVGA card, they have a 90 day free trade up program.
Z97 and 4790K is a good combo.
But with skylake now available, I think I5-6600K and Z170 is a better long term choice.