If you upgrade now you will see a performance increase but you may be disappointed with how much that increase will be.
The thing that's drawing you is the Pci-e 3.0 and as of right now the cards that are Pci-e 3.0 like the 660Ti will not saturate a Pci-e 2.0 slot so it will be a few more upgrades in video cards before we see some benefit from the Pci-e 3.0.
The computer that you have now is still going to give you good gameplay and good fps. If you were looking to upgrade for some gain in performance then you can consider a new video card like the 660Ti and leave the rest the same. Haswell is going to be coming out soon and you can take a look at that cpu and consider an upgrade on the cpu and MB then. But in the meantime a new video card could be a good choice.