i got a 550ti the other day for my pc, can nearly run bf3 at max without much issue.
playing on medium with 60fps constant for better gameplay
on a similar rig to you aswell, xept my cpu is an amd phenomII x4 945
runs fine on my 580W psu, but a bigger one wouldnt hurt for the future.
doesnt really matter which card you get, you'll be satisfied by either one.
550ti was the same price as an amd 6850 over here, i chose nvidia because i wasnt happy with the AMD drivers on my 5770