At only 720p I'm surprised the 3.0GHZ Athlon II X3 is struggling. And most games make little to no use of the 'missing' L3 cache. In fact, now I'm even surprised the GTX 550ti is struggling at that rez. How and why are you capping the frame rate? And why at 40 fps?
Have you cleaned your system of crap that wastes Windows resources, and registry errors that cause slowdowns lately? If not, try CCleaner: http://www.piriform.com/CCLEANER
Do the Clean and Registry both.
When you installed your GTX 550ti, did you remove the old driver first?