Upgrading graphic card on Mac Pro 5.1

Depends. How much power?

Fastest slot-powered:

Without bootscreen: RX 560 (A 75W 14CU variant without a 6-pin connector by not-XFX)

Fastest single 6-pin:

With bootscreen: Quadro K5000 Mac Edition

Without bootscreen: Sapphire Pulse ITX single fan RX 570

Fastest 2x6-pin (or 1x8-pin)

With bootscreen: GTX 680 Mac Edition (or flashed) [Edit: The 7950 Mac Edition might be even faster]

Without bootscreen: RX 580 (excluding some 8+6 pin super clocked variants)

For reference: https://support.apple.com/en-us/HT208898

If you could find it a Mac Radeon HD 7950...

https://www.ebay.com/i/254419767369?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=254419767369&targetid=859829361278&device=c&mktype=pla&googleloc=1013542&poi=&campaignid=6470742440&mkgroupid=88656158866&rlsatarget=pla-859829361278&abcId=1141156&merchantid=114719204

Though many opt for the Radeon RX 580...

https://eshop.macsales.com/item/OWC/MP1012R580V/?utm_source=google&utm_medium=shoppingengine&utm_campaign=googlebase&gclsrc=ds

Pther cards...

http://www.macvidcards.com/store/c16/MacOS_10.14_Mojave_Compatible.html

Great info, thanks! :)