You have a very nice setup now.
Most of us game on the primary monitor and use the side monitor/s for relatively static information.
The side monitors will not hurt gaming performance.
Multi monitors are great for desktop work.
I might consider a larger monitor for more immersive gaming.
OTOH, if you are looking into triple monitor gaming, that is another issue.
If you are multithreading to a great extent, you need sufficient ram to hold all of the tasks going on.
You have 8 threads which should be sufficient.
Have you overclocked your "K"?
If not, you are leaving some 30% cpu performance on the table.
My reading says CSGO responds to single thread speeds.
As of 12/04/2016
What percent can get an overclock at a somewhat sane 1.4v Vcore.
I7-6700K
4.9 5%
4.8 21%
4.7 64%
4.6 96%
Here is my stock approach to the cpu vs. gpu upgrade question:
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 a gpu upgrade is what you need, your 650w psu will handle any modern graphics card.
If you need more cpu power, you are looking at a modern 9th gen intel or ryzen rebuild.
That will need a new motherboard.
If you want more ram, there is a potential issue.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.
If you do buy more disparate sticks, they should be the same speed, voltage and cas numbers.
Even then your chances of working are less than 100%
I might guess 90% success for intel and less for amd.
What is your plan "B" if the new stick/s do not work?
If you want 32gb, my suggestion is to buy a 2 x 16gb kit that matches your current specs.
Then, try adding in your old 2 x 8gb,
If it works, good; you now have extra ram.
If not, sell the old ram or keep it as a spare.
How full is your ssd C drive?
If it is near 90% full, it is time to increase the capacity.
A SSD will lose performance and endurance as it approaches full.