This is not a PC. Leave your PC thinking at the door and see what the OWC (see google) folk think what cards work in that machine.
http://www.everymac.com/systems/apple/mac_pro/faq/mac-pro-default-graphics-cards-dvi-dual-link-mini-displayport.html lists a few but you may have to check if that's the right model.
Unlike PCs you stick to the lists or boards that folk tested.
I have a mac pro mid 2010 5,1 that needed a better GPU to run resolve. I got a GTX780 ti but the Mac screen is black once its turned on. I realise I may not be in the right area for this but I'm desperate at this stage. Why is this happening, and once I get it ready, will it be compatible with resolve? I talked to a guy who mentioned openCL and Cuda? Bit lost..... Any help appreciated!