I think you should consider overclocking your CPU first.
You can probably reach ~ 3.5 GHz if it is x4 920 CPU.That is in my opinion minimum CPU frequency for some sort of comfortable gameplay, and 4 GHz recommended for good gameplay.
Maybe this will help, and I hope you have some aftermarket cooler.
http://www.xbitlabs.com/articles/cpu...locking_3.html
Your GPU is enough for almost everything maxed out on 1680 resolution.
(I have gtx560Ti but with 2500k)
Upgrading with i5 2500K would be a good thing.