Well, The best cpu you can put in that motherboard is an i7 7700k, which is bad, since a ryzen 3 3300X is the same, and will cost less, even with a motherboard.
Theres no real upgrade path for that pc.
Unless you actually get a good deal on an i7 7700k, but even then, that would close you off for upgrades.
Getting a new gpu will give you much higher performance in some gpu intense games, but might have 0 impact on performance with cpu intense games.
if you just wanna squeeze more performance out of it, get a new gpu.
if you want a better upgrade path for the future, and a little bit of performance today, get a new motherboard and cpu.
(probably a ryzen 3 3100/3300X and a b450/b550 motherboard would be your best bet)