How to improve graphics display iMac 27" 2020

Are you asking for faster image processing or frame rates or rendering? You could purchase an eGPU. Use an eGPU with your Mac - Apple Support

Are you asking for better display image quality? Purchase an external display to use as a second monitor. However the Retina 27" 5K is already one of the best on the market so finding something better would be a challenge.

I have just ordered additional RAM to improve performance

I hope you have not spent money needlessly.

Exactly what performance boost is needed? You computer woulds have Apple's fastest storage, the SSD, so it should not be feel slow unless you have been advised to install needless anti-virus apps, so-called "cleaning" apps, or a client-side VPN.

Except in rare cases, adding RAM does not make a performance difference detectible at the keyboard.

The graphics hardware in your iMac is soldered to the logic board and cannot change without replacing the entire logic board. The options with your model were:

4 GB (Radeon Pro 5300)

8 GB (Radeon Pro 5500 XT, Radeon Pro 5700)

16 GB (Radeon Pro 5700 XT) GDDR6

None are slackers.

So details about what you see that needs to improve will greatly help us in this setting where we can neitehr see nor touch your computer. Otherwise, all we can do is post generalizations.

Even the 4GB model (i have that) should give excellent dispaly quality.

You already have one of the best displays out there. You can always change the resolution smaller if you want more detail but may be hard to read.