Gaming mostly depends on a good graphics card.
There is no such thing as "bottlenecking"
If, by that, you mean that upgrading a cpu or graphics card can
somehow lower your performance or FPS.
A better term might be limiting factor.
That is where adding more cpu or gpu becomes increasingly
less effective.
Good luck in finding one at anything < 2x msrp, if you can find one at all.
GTX1060 6gb is a decent graphics card if you can find one at a decent price.
Here is tom's gpu hierarchy chart to guide you in the value of different graphics cards:
https://www.tomshardware.com/reviews/gpu-hierarchy,4388.html
Yes, GTX1660 super is better, but at what price.
Since you are buying used, I have no ideas as to what you could get.
Here are some thoughts on your parts.
Why buy an older R3-3200g?
You are paying for integrated graphics which you do not plan to use.
It is no great shakes as a gamer either.
It has only 4 threads and a passmark rating of 7232. That is when all 4 threads are 100% busy.
The single thread rating is 2230. That is what games need most
https://www.cpubenchmark.net/cpu.php?cpu=AMD+Ryzen+3+3200G&id=3497
By comparison, a similarly priced i3-10100 has 8 threads and a rating of 8949/2648.
Here is a gaming review of the i3-10100:
https://www.techpowerup.com/review/intel-core-i3-10100/15.html
Considerably better than even the 3400g.
Intel does not depend on fast ram for performance. 2666 speed is the fastest you can use.
Any lga1200 motherboard will do.
What is the make/model of the psu?
You only need about 450W.
Nothing wrong with much stronger, it will only use the power demanded of it, regardless of the max capability.
Platinum rating, by itself does not indicate good quality. A 7 or 10 year warranty might.
Quality counts. A cheap psu can become expensive if it fails under load and damages all of your parts.
Here is one list of psu quality. Look for tier 1/2, no less than 3:
Credit to @Starelementpokeand @Energycore for helping out with this long-awaited refresh of the old PSU tier list by Aniallation (who previously went several months without logging in), leaving the old list in need of help. This is simply an update of the list that was flawed in creation by the P...
linustechtips.com
Today, do not go cheap and buy a SSD and not a HDD.
It will make all the difference in the world as to quickness.
New, a i3-10100 is about $110, a lga1200 motherboard is $75, and a 2 x 8gb DDR4 ram kit is about $75. Total $260. What does your $400 budget need to pay for?