I would go for a 955 rather than the 965, cheaper and still has the unlocked multiplier, take the money you save and get a heatsink like the cooler master 212+ and OC it.
As for the GPU, what is your screen resolution? A 5770 is about as powerful as you can get on your current PSU, a 5850 would give a far more significant boost in performance but really should be run on a more powerful unit than a vx450.
is there much off a difference in the 4850 vs 5770? also my screen res is 1366 x 768 right now. but i plan on getting a better in the future. girlfriend got me this one as a present so i didnt bother spending extra money on a monitor. what resolution would the 955 or 965 and the 5770 be playable at