Installed a MSI Radeon Gaming X RX 580 in to the Mac Pro the other day. You need to buy a new harness for most older macs for this to covert from two 6 pins on your board to the 8 pin on the 580, it's $12 on Amazon or cheaper on eBay.
One caveat, the card does not like converter cables, I decided to use on only one monitor and mouse connected at startup and got a bright pink screen. I bought the card used from a bitcoin farmer who ran it only for a couple months on Sundays
before giving up, so I thought it was shot at first. After searching the internet for the answer eventually one guy had posted the same issue and it turned out to be the HDMI to DVI cable he was using, going straight HDMI to HDMI no issues. Fixed! Weird that it's not as copasetic with it, but with DVI, 2 HDMI, and two Display port connections on it I'm OK with that. On the plus side, people have reported the Saphire and other cards are slightly larger than their PCIe slots, making installing the cards a PITA, fans hitting your card in the neighboring slot etc. MSI kept this in mind it seems and the card isn't wider than the slot, including the fans. No install issues.
This thing is ridiculously fast, 16X the memory of the old card, Twice the frame rate in Cinebench, overkill is good sometimes. Now to install Mojave.
