I'm saying your CPU will not bottleneck a card up to a GTX 560 Ti. Anything over that and a new CPU or at least an overclock of what you have would be needed. Once again resolution is important. At lower resolutions the CPU is more important and at higher resolutions the GPU becomes more important.
The GTX 550 Ti is an entry level card at best. The first thing I would do in your situation is upgrade the graphics. Unless you plan on building a completely new computer.....motherboard, CPU, RAM and GPU.