Sapphire Radeon HD 6570 in Mac Pro 3.1, it works!
It was just "Plug n Play", no messing around needed. Have to clarify that I am using El Capitan but it should also work with Snow Leopard. No bootscreen as expected in the Sapphire, but that's the reason also got the 2600XT.
I felt a difference when using Photoshop and the animations in OS X, worth the few quid!
I just posted this because I couldn't find much information in the internet about compatibility, this may help some people to decide which card to install.