Let me start by saying: stay away from AMD cards. Second, I used to have an NVidia GTX 570. It is excellent. I recently upgraded to a 580. That's as good as it gets unless you were to buy the multi-thousand dollar Teslas or Quadro 6000.
It is a great time to get an NVidia right now because the prices have come down because of the arrival of the GTX 680.
First and foremost, most GPU accellerated plug-ins use NVidia's CUDAs. Therefore, they do not support the AMD cards. Second, I have used both a Radeon HD-5770 and HD-7970. Both of them gave me trouble. I was particularly that the 7970 gave me so much trouble because it was fantastic for playback.