What GPU can i put in my Mac Pro 5,1?
Apple has an &quot;official&quot; list of graphics cards that are compatible with the Mac Pros  (Mid 2010).
The ones specific to the 5,1 are:

Mac Pro (Mid 2010) and Mac Pro (Mid 2012)

ATI Radeon HD 5770
ATI Radeon HD 5870


Now, according to EveryMac.com, there are several more adapters that are compatible:

NVIDIA Quadro FX 4800

NVIDIA Quadro 4000

AMD Sapphire Radeon HD 7950*

EVGA GeForce GTX 680

PNY NVIDIA Quadro K5000



Please note that the Sapphire Radeon HD 7950 only is compatible with
the &quot;Mid-2010&quot; and &quot;Mid-2012&quot; Mac Pro models. The other four video
cards above are compatible with the &quot;Early 2008&quot; and &quot;Early 2009&quot; as
well as the &quot;Mid-2010&quot; and &quot;Mid-2012&quot; Mac Pro models.

Personally, I have used the  AMD Sapphire Radeon HD 7950 because it wasn't just compatible with OS X (10.7+), but Windows Vista (which meant it worked in 7 and should work in Windows 10)  and FreeBSD as well.
I am currently refurbing a Mid-2012 Mac Pro (side project) and I fully intend to put this exact card in the mix.