This might surprise you but I'd suggest an NVIDIA GTX 285 if you are going for a Mac compatible card. It sounds like you want to move into the Davinci world and if you do this is the best card on a Mac. They can be a little hard to find but we managed to pick up a couple of the Mac versions of these cards on ebay. I think we paid about $500 per card. 

We run several Davinci Resolve systems here and the GTX 285 is faster than the Quadro 4000, even though its a newer and supposedly faster card. I purchased the 4000 thinking I'd up my performance and soon realized my real time performance was reduced in Resolve, less nodes of correction with the 4000 and went back to the 285. 

You can install the Lite version of Resolve for free. It is fully functioning unless you decide you want to work at resolutions beyond HD for renders out. Otherwise Resolve is a fully functioning application, which still blows me away. You can contact me off the list if you need any further information. We have a lot of experience with Resolve systems.