You will be able to get a much better answer to this after CC is released in a few days I suspect.
Up until CC, for PPro versions from CS5 to CS6, AMD cards (except for maybe some very special cases - i.e. Macbook Pro) do not give MPE what it needs but nVidia does.
>3GB video card
My VERY general understanding is that a card with more than 1Gig of video ram is "mostly" useful if you are editing LARGE format video... such as the 4k codec (aka Red ???) where each individual video frame is very large
Comparison picture of video screen sizes http://forums.adobe.com/thread/1120039
As far as the new (June 17) CC version and cards... I will GUESS that a "faster and more cores" AMD card will be what you want, the same as nVidia today
Since the GPU ram has to handle both the frame data and the data for CUDA processing, having more GPU ram is always a positive. Right now only 2k+ or Red media seems to really push the frame buffer past 2GB on the cards. That very well may change with CC but that is yet to be determined. Open CL will be in a relatively immature development stage with the coming version. I have yet to see Open CL outperform CUDA. I would not expect that to change with CC so plan on Nvidia. The biggest factor to GPU acceleration is memory management. Adobe has had time to refine that with Cuda but not with Open CL. That alone will be a major performance factor.