The 6770 is outdated so it's between the gt 640 and the 7750. They are pretty much the same in performance, personally I would go with the gt 640 because I'm an nvidia fanboy
Here's a good article on Toms, which should help. Seems the 7750 is a lot better in most games, up to 40% approx!
The system spec is similar to your's so it should be a good guidline for you!
Happy gaming. Nice system
Actually just re-read your post, and saw you question isn't related to gaming. Couldn't find much on application performance with photoshop etc. Sorry for the mis-info. Although the 7750 is better gaming wise, not sure how that translates though!