For this particular type of work, I'd say that HD 7770 would be a better option.
It's less powerful than HD 6850, but not by much.
It eats less power than HD 6850 and has newer technologies support.
For gaming HD 6850 would be a better choice, but for work - I'd say HD 7770 is a better choice.
Just don't forget that Photoshop mainly depends on your drive (it would work MUCH better on the SSD, rather than on the HDD) and your memory.
If you want a truly high performance during work with Photoshop, you should think about getting high-frequency (at least 1333MHz. I'd say that 1600MHz would be more than enough), high-capacity (at least 8GB) memory, and about getting an SSD, not about getting video card.
Getting even the most high-powered video card won't help much with Photoshop, in this particular case you should think about memory and SSD.
P.S. Why do you even need something like HD 6850/7770, when you already have Sapphire's HD 7950 (one of the best cards on the market right now)?
And if the GPU is not important at all, I could get a lower card and put more money in the SSD as it will be a work-only computer.
Don't get my words wrong: yes, I did say that GPU doesn't matter much when you use Photoshop, but IT STILL DOES MATTER, even though not on a big scale.
You're going to use CS6, which is quite of a beast, and even though memory and SSD matters more, than the video card you have, you'd still benefit more with a powerful card, than with a very cheap and weak one. What I'm trying to say is: moving to some card that is weaker than the ones that's mentioned in this thread, would be quite of a mistake, I'd even say that HD 7770 is the lowest point you can get with CS6, you'd better not get anything below that level (HD 6850 is alright, but HD 7770 would still perform a little bit better).
About the SSD: I've read that thread and I agree with others on that Intel 330 is a better option.
But, in my personal opinion, I'd still highly recommend getting Intel 510 (if you could find it somewhere near you and can afford it, get it without any hesitation at all, it's THAT good).