The more effective upgrade will depend on the types of games you will play.
Some games are graphics limited like fast action shooters.
Others are cpu core speed limited like strategy, sims, and mmo.
Multiplayer tends to like many threads.
You need to find out which.
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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 or more cores/threads. 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 threads to less than you have.
This will tell you how sensitive your games are to the benefits of many threads.
If you see little difference, your game does not need all the threads you have.
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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If you think you need a stronger processor, it is likely that replacing the i3-7100 on your existing lga1151 motherboard will be a more economical upgrade than buying both a 2600x and a new motherboard.
Your i3-7100 has a passmark rating of 5783 and a single thread rating of 2228.
The rating for the 2600s is impressive at 14350, but the single thread rating of 2138 is about the same as what you have.
It becomes important to know just how much you will get from added threads. Most games, but not all, will effectively use only 2-3 threads and the performance of the single master thread becomes most important.
If you need more threads, the 8 thread i7-7700 will run on your motherboard.
It has a rating of 10753/2349.
On the graphics side, the question is easier.
A GTX1060 6gb is a nice upgrade. It will only need a 450w psu to run.