I am looking for a readily available Graphics Card for a Late 2013 MacPro. I would ideally like 4GB on it and I am not sure about doing a dual or a single.
I would also like to make sure the card has updatable drivers and firmware.
Found this Apple Website that shows what came with the MacPro originally and what can be GPU card can be upgraded to from 2017: Cylinder Mac Pro Default and Optional Graphics Cards: EveryMac.com
According to this Apple Website only Three AMD GPU cards were offered to be compatible for the MacPro which was Apples intentions: The Definitive Trashcan Mac Pro 6.1 (Late 2013) Upgrade Guide
As to having updated Drivers and Firmware you are going to have to ask Apple Support since AMD doesn't have any drivers for the Dxxx Firepro GPU Cards. Apple is the one that creates the drivers for the AMD Firepro Dxxx Professional cards.
If the MacPro has a Thunderbolt 2.0/3.0 port you can purchase an External case in which you have more options in GPU Cards to install and run.
Otherwise it seems you are stuck with three expensive AMD GPU cards as replacement or purchase a newer version of Mac Pro according to this Apple Website: Mac Pro - Technical Specifications - Apple
If you are talking about trash can Mac Pro, you'll need to go for external GPU
It's doable, but some works need to be done
1. Apple doesn't officially support eGPU through TB2. However, there's a workaround
2. At this moment only AMD/ATI cards are recommended since nVidia cards have been out of the game since 10.14. Unless you want to use some really dated but native supported Kepler cards.
From my personal experience any card after Polaris(Vega, Vega20) works perfectly.
Just keep in mind if you want to use one from XFX, you might need to flash other vendors' vBios so it will work.