Help needed re upgrade graphics card Dell 9020 SFF

[NewGamer76]

My 16yo son has a Dell 9020 SFF PC he uses for gaming - he tells me the graphics are not great and he would like to upgrade the graphics card and memory.
I have looked and looked but I'm just getting confused, particually around the graphics card.

The current PC specs are: Intel Core 15-4590 @3.30GHz. 8GB RAM

Can anyone recommend some graphics cards that will fit and that will work with the existing power unit? I would like to buy someting that will make a noticable difference, otherwise it will be a very disappointing Christmas present for him!

Huge thanks in advance

 

[NewGamer76]

Thank you for your quick response.

his monitor is :

AOC Gaming C27G2E - 27 Inch FHD Curved Monitor, 165Hz, VA, 1ms, Low Input Lag, 1500R Curved, G-Menu, Tilt (1920 x 1080 @ 165Hz, 250 cd/m², HDMI 2.0 / DP 1.2)​

we really dont know what FPS we need.

Appreciate that the machine is old. I dont want to buy a GPU and find out there is not much difference. I just wondered if it would suffice for a year or so before we start investing in building a proper gaming pc ... they just seem so much £££. Maybe we just need to look for second hand ones.