I have a GTX560Ti running the latest drivers and it works perfectly with CS6. I assume the later model will too.
I also have a GTX 560 and it works very well with CS6. That being said you can get a 660Ti for about $50 - $60 more. You have to make sure you have a pci express 3.0 slot to put it in. the 560 will fit in a pci express 2.0 slot. (Also if you go for a 560 or a 660 go for the Ti version and also see if you can find the "super clocked FTW" version as they have higher clock speeds, more cuda cores and more memory... they are basically like getting the stock 570/670 series card if not even outperforming it.
Also one thing that photoshop CS6 relies heavily on is your hard drive. I was running into big performance issues with my system as my hard drive was beginning to experience I/O errors. My old version of CS3 seemed to ignore these issues and would run fine but CS6 would lock up on me on a regular basis. I think that it might take advantage of your swap files more aggressively than previous versions. So if you have an old hard drive I would also look into upgrading to a new one. Preferably a large SSD as that will run much faster. However I bought a seagate baracuda 3tb 7200 rpm HD and now my CS6 runs very smoothly with my 560 ti
These cards are all FAR more powerful than is really needed by Photoshop to do 2D work. Any card that scores more than 1000 on the G3D test on the following site will run Photoshop quite well:
You might find some advantages with Photoshop 3D work, and possibly a bit more future-proofing, by using the more powerful cards, especially those with more VRAM.