My brother’s Mac Pro is single CPU, Quad-Core 2.8 GHz Xeon W3530, Mac Pro with 32 GBs of RAM, and a hand-me-down GeForce 1060. He'd like to upgrade it so he can put off buying a new computer for a year or two and didn't want to spend much money.
So he purchased an AMD 6600 XT and 6-Core CPU for all of $225. This is the process and the results.
XFX Speedster SWFT 210 Radeon RX 6600
https://amzn.to/4aLK2Ti
RX 580s are cheap again
https://amzn.to/4gntAK7
The $75 Upgrade to a Mac Pro 3,1.
https://www.youtube.com/watch?v=XaStlvmvluI&feature=youtu.be
Important links:
AMD VB Flash utility
https://www.techpowerup.com/download/ati-atiflash/
Syncretic's AMD 6000 series ROM patcher
https://forums.macrumors.com/threads/amd-radeon-rx-6800-xt-6900-xt-for-macos.2262295/page-37?post=31045379#post-31045379
MacSound Solutions: How to flash a RX6600XT RX6800XT RX6900XT for Mac Pro 5,1
https://www.youtube.com/watch?v=GP6tIwezz2I