It seems RX 580 cards had a recent price drop. Or they're being cleared out. Newegg
has them for as little as $150 (after a rebate). I seem to remember them running $200+.
These are good upgrades from all cards shipped with the 2009-2013 Mac Pros, and exceed both retail upgrade cards (Radeon 7950 and GTX 680). They should also work in eGPU cases for MB / MBPro with thunderbolt jack, running 10.13.4 or higher. They typically use a single 8-pin power connector.
Beware that the RX580 is offered in 4GB and 8GB VRAM configs. I'd avoid the 4GB cards.
I picked up a triple-fan Asus for $170 ($180 - instant $10 code). It's a 2.5-slot thickness, meaning it covers the next PCIe slot. But I valued the low noise more than the next slot. Oversize heat sink + triple fans = next to no noise. It's also possible my 7970 needs new heat-sink grease - it seemed quieter before. But never as quiet as the RX580.
If you need a true 2-slot width card, most of the 2-fan designs are good. Check the pictures for one looking at the ports plate, to confirm it doesn't overflow the plate on the right.
These cards have PC BIOS, so Mac Pros will not show a boot screen until about 50% through the boot process. No EFI boot selector while holding down the Option key. Keep your old video card handy, in case of a boot issue. This is not a concern for eGPU cases, as the eGPU does not load until late in the boot process anyway.
Full-screen YouTube no longer maxes out my GPU, and the card fans are almost always silent. ie - I'm only hearing my case fans. This is with dual 4K monitors. Pretty screen savers used to max out the fans on my 7970, now the RX580 stays almost silent.