Your i7-7700 is still a good gaming processor.
Likely, a graphics card upgrade would be best, particularly if you play fast action games.
Your Seasonic 620w psu is excellent quality and will support even the strongest of upgrades.
I might suggest something like a GTX1660ti.
Intel does not depend on ram speed for performance when using discrete graphics.
No need to change that. 8gb is ok so long as you are not multitasking while gaming.
Past that, you can upgrade in any order you want.
A SSD will make everything you do feel snappier.
I like the Samsung EVO. If the used portion of your 1tb HDD can fit on the ssd, Samsung supplies a good C drive mover app.
Take your time on the monitor.
Save up for a really good one because they are relevant for a long time.
If you can see one in person, that is best.
I would look at larger/wider units.
Look for a 4k monitor, they are getting cheaper and you can always dial back the resolution.
IPS type panels with 178/178 viewing angle will look best, but those with tn panels will be faster.
Pick tn if you value fps over all else. The value of g.sync/freesync and such is probably not worth the extra cost. there are driver buffering solutions do accomplish much of the same frame matching functions.
If your ram is a single 8gb stick, you could take a chance and buy a second matching stick.
Proper operation is not 100% guaranteed, ram must be matched.
Likely your odds are perhaps 90% of success.
Not bad assuming you can return your purchase if it does now work nice with your old ram.
Here is my stock answer to this perennial 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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