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.