What Graphics card should I buy?

Your selection will depend on the purpose of usage.  For example, you would want a GTX for gaming or a Quadro for video, graphics works, or an RTX for the latest features as well as the performances.

For system fitments, the selected card must be a low profile (half height), with power draw within 50w, although cards with up to 75w have been working for some users.  And last but not least, must be a single slot card.  While some users have tried double slot card on the PCIe x4 slot, it's not recommended.

Once you have the list narrow down, taking the budget and performance value into consideration before making your final selection.

Chino has hit all the key points already.

just to add for gaming best gpu might be low profile GTX1650 installed in pciex4 slot which has open end in 9020 sff

It needs to be stated that 75w cards for the 9020 SFF are outside of Dell specs.  While 75w cards in SFF models work for some on YouTube, the YouTubers never seem to say how long it lasts.  I don't mind if some did, and there's examples, then I'll stand corrected.  Looks like formatting below isn't working right yet.

Some suggested cards:

• 30w GT 1030 GDDR5
• 30w-47w Quadro's P400, 600, & 1000
• 50w RX 6400

@Chino de Oro​ For a 2013 era machine, the best is the Nvidia Quadro K2000 which cost over $600+ back in 2013 you can find it for under $30 these days.

OP has an SFF, small form factor chassis.  Be mindful if the suggestion is taken seriously.

A Quadro K2000 was not and is not the best in 2013 or 2023.

Currently, best low profile, slot powered is RTX A2000.