Video graphics card for editing

Well if you are using Adobe Premiere Pro CS6 then 'generally' Nvidia to utilise the Cuda acceleration (they have a list of cards and info there). Some AMD acceleration is supported under OSX and I imagine with OpenCL gaining traction they'll start to support that a bit more.

If there is specific software you have in mind see if they have anything to say about it.

CUDA is more focused on things like color correction and the Mercury Playback Engine (which I believe is now supported/accelerated on AMD chips these days) --- OpenCL/GL acceleration is certainly showing an impact on video encoding and editing these days, and will only expand in the future.

Check out the AMD FirePro V4900 for a great entry-level workstation card.

Here yah go in Vegas -- there is CS6 "Premiere' OpenCL/GL stuff out there, too, to look for ....