I will be gaming as well as Photoshop and rendering videos. The only game I will basically play is Call of Duty 4. If I go with the 5850 set up, I will not crossfire the 5850. If I get the 5770, I will crossfire it about a year later. I will be gaming at 1680x1050.
Which set up would be better?
Also, what fps do you think I could get with a single 5770 at 1680x1050 in Call of Duty 4?
The 5850 is going to make for a better gaming experience, but the PII 955 will be better at rendering and PS. What's more important to you? You "may" be able to unlock the 720's 4th core, but nothing is guaranteed. A 5770 will run COD4 very well at 1680x, but I dont have specific values on hand.
If I were you I would definitely get the 720 + 5850, but that's just my opinion.