There could be several factors to consider. What resolution are you trying to game at? What C2D processor do you have? The dual core is a bit slow for even a 560ti to get its full benefit.
The bugs and errors are another subject. Is your system kept clean of registry errors, junk files, and other resource wasting crap? If not I would run something like CCleaner and do the Clean and Registry both. Its free. http://www.piriform.com/ccleaner