Hi, At the bottom of this FAQ, you will find a list of tested videocards wih CS6: http://forums.adobe.com/thread/979969?tstart=0
EDIT: when I googled "tested videocards Ps CS6, I got this help document, the list seems to match the other one: http://helpx.adobe.com/photoshop/kb/photoshop-cs6-gpu-faq.html
It seems it's NEVER possible to just say "get this model" because all of them have strengths and weaknesses.
I develop GPU-enhanced software (not Photoshop, but plug-ins for Photoshop). My experience has been that ATI's driver quality is higher than that of nVidia. I have to code more exceptions in my software for both nVidia and Intel than ATI models.
That said, of course "past results are no guarantee of future performance", and ATI seems to have been stumbling lately with their last few driver releases. Catalyst 12.2 (from this past March) seems to be the golden version to get.
If I were put on the spot to recommend a video card right now at the moment, I'd probably suggest the VisionTek ATI Radeon HD 7750 model. I have been using exclusively VisionTek brand cards myself and I've had ZERO problems with them. I don't personally have the model I mentioned (I have a 5670 in my main Photoshop workstation now myself, though I'm tempted by the new 7000 series), but I know someone who does. Keep in mind video cards have different power supply demands; the card I mentioned is a low current-draw model (the 7750 doesn't even require a separate power supply connector), and low current models are quieter and don't make as much heat.