http://www.overclock.net/t/275374/evga-780i-overclocking-guide
Try this it guide it will be extra easy as you have a QX unlocked chip.
3.6 ghz should be doable but make sure you have aftermarket coolig, a tx3 for 15 bucks would do the trick to 3.6Ghz and thats likely the limit of the board.
You will really want to try this before you move on. at stock tyour chip is in the 4th tier of the gaming chip guide, if you can get it past 3.6 it would be a good thing move you up a tiers worth of performance possibly 2 depending on how high you get.
the chip is very likely capable of getting to 4Ghz just not on the nvidia motherboard they are not known for the best overclocking stability with 4 cores. 3.6Ghz quad is plenty fast to put up 60fps in a lot of games like most gaes frankly with exceptions ofcourse but get a new board and stuff when you have abetter card. right now you are missing out on dx11 so i woulld look at getting the rest of the machine after the new card.
it may even be enough.
There are reviews showing performance with a core 2 quad vs the new cpu`s and it`s not doing badly at all andd it`s only clocked to 3Ghz
http://www.ocaholic.ch/modules/smartsection/item.php?itemid=817&page=7
I love this article it`s keeping with what I have.