Ugh, a B3 Q6600 and a 680i based motherboard.
Sorry, I doubt you will be able to get your CPU past 2.8Ghz.
I would skip upgrading anything at the moment.
If you can get a bit of an overclock out of the CPU great!
If not, it is still pretty powerful as it is.
Your current 8800GT SLI is roughly as powerful as a GTX 280.
Especially considering your resolution, upgrading your graphic card(s) would make no sense.
Better would be to upgrade to a nice 24" display and save for an ATI 5 series card.
You are right, my Q6600 B3 does not OC well. And yes, I am running the CPU at stock speed.