I got a GTX 560 TI with a duo core 2 6850 3ghz and 4gb ddr 2 memory.
I can rum Planetside 2, crysis 1-2, Shogun total war, deadspace, skyrim, battlefield 3 etc, but this is with lowerd graphics, mostly shadows as it takes a huge toll.
Now your cpu is quad and should perform better than mine but not 100% sure about that.
Any way if you cant play games like that then something is most likley wrong. Dont have to be hardware related thoug.
If you haven't reinstalled OS in a while i suggest you do that, as many software spyware, and antivirus software can slow your performance by 70% atleast.
Install fresh win 7 or XP, keep away from vista.
Install 3dMark and do some tests and see how you compare to others with the same specs.