a pc 970 will kind of work in osx 10.10+ with nvidia's driver with some problems (but will work)
if you want some advice post which mac pro you have, version of osx you will run (just 10.6?) and what you want to do with the card apps (name apps) or games.
edit last nvidia mac edition was a 680. but if you want a 680 your better of getting a pc one as there cheaper on ebay and will work.