Upgrading from HD4850

[maor66]

After playing BF3 with my 4850, I saw that I will probably have to upgrade my card.
CPU is Phenom x3 720@3.5GHz, what is the best GPU I can buy that will not cause bottleneck?
Also my PSU is Cheiftec 550W if it matters.

 

[maor66]

1680*1050
for now, just tell me the best card that won't bottleneck.

 

[Emperus]

Either of the HD 6950 or the GTX 560 Ti should be a healthy upgrade.