I've got a couple of 5,1's. A 3.46 hex and a 3.2quad. I've had a flashed 680gtx that i bought unused 3 yrs ago for about 300usd and its been in my primary mac pro (the 3.46). I bought another used 680gtx (100usd) for the second machine to replace the 5880 but ended up selling it to a customer who wanted mojave compatibility. I could not find another and was not inclined to spend 200usd on another flashed 680gtx. So... I bought an nvidia rx580 sapphire from amazon for 200usd. At least its a fairly recent model ! pretty sure its the apple recommended version. Its GREAT. powered by a 2x6pin to 8 pin adapter and its dead quiet in anything i am likely to use and a bit faster than the 680gtx which i am moving to the second machine... as my boot drive is a 970pro nvme drive i do not have to wait long for a boot screen, which kicks in after the drivers are engaged.
The second mac pro has bootcamp but my primary machine is just running mojave 14.4 and has bootrom 141.0.0.0 (as of yesterday). Anyways because i am not a gamer or encoder of videos i went with the recommended card and it seems very compatible. If you have bootcamp, however, as i do on the second machine, you should run a flashed card because if your drive selection gets stuffed up you wont be able to option boot to switch systems
The second mac pro has bootcamp but my primary machine is just running mojave 14.4 and has bootrom 141.0.0.0 (as of yesterday). Anyways because i am not a gamer or encoder of videos i went with the recommended card and it seems very compatible. If you have bootcamp, however, as i do on the second machine, you should run a flashed card because if your drive selection gets stuffed up you wont be able to option boot to switch systems