Adobe forum: Photoshop CS6 and the GPUOne guy's blog
on video card performance with CS6
Short analysis: get a video card with at least 512MB of RAM. If you work with large photos and effects, then go for 1GB. If you deal with 1-2Mpixel pictures, just about any video card will do. If you work with 64Mpixel pictures and high volume workflow, then a gaming card in the GTX660 or HD7850 class or better will provide max performance.
I'd recommend the HD 7750 over the 6670 - it uses less watts and has double the processing power...of course it's ~$20 more for a passive version. The downside to AMD is they don't downclock the VRAM when idling or in 2D modes when you have 2 monitors...so it burns 5-10W more than the equivalent Nvidia card. On the Nvidia side, I'd go for a GTX 650. Nothing passive, yet...but there are some quiet cards.