Turning Secure boot OFF in BIOS and installing 3rd party card should be fine.

Lowest I would go is GT1030

https://www.amazon.com/Gigabyte-GV-N1030OC-2GI-Nvidia-GeForce-Graphics/dp/B07646VQ6T/

There are better cards but they are more than 2X more expensive.

https://www.amazon.com/Gigabyte-Geforce-GDDR5-Graphic-GV-N105TOC-4GD/dp/B01M25X363

https://www.amazon.com/GIGABYTE-GeForce-Graphics-Windforce-Gv-N1650OC-4GD/dp/B07QHGKC2D/

Better than those becomes very expensive.

The cost of GPU goes up to hundreds then thousands of dollars.

https://www.amazon.com/Nvidia-Geforce-Super-Founders-Graphics/dp/B07V2V59LF

https://www.amazon.com/PNY-GeForce-Gaming-Overclocked-Graphics/dp/B07GHXQW3Z

https://www.amazon.com/PNY-VCQRTX6000-BLK-Quadro-RTX-6000/dp/B07HYC7P75

You will not be able to determine your available upgrade options without knowing if you have the base 460w psu, the 850w psu, or if the prior owner swapped out the oem psu.  That's the first thing you should check before looking at new gpus.  

Go here, enter the service tag on the back, and lookup what the pc originally shipped with: https://www.dell.com/support/home/en-us

Then look at the side label on the psu to verify it hasn't been replaced from the oem Delta or Huntkey model.  The 850w model is modular, the 460w is not.

Forgot to mention, the other thing to know:  check Device Manager / Display Adapters to find out your current GPU.  

What would be the easiest PSU to replace the original 460W item