read this article it will help you a lot, you need to understand how thinks work if you want overclock your system and keep it stable.
i7 920-i7 930-i7 950 basically it's the same CPUs. the thin is that 950 has higher stock clocks which is 3.06 HGz. i7 950 overclocks to 4GHz with much LESS voltage than 920 and 930 but if you want to hit over 4 GHz the voltage will be about the same compare to 920 and 930
it's gonna be a bit harder to overclock and keep your system stable with all your 12 GB of ram. if you build your rig for gaming, movies, web.... for basic stuff you do not need 12 GB of ram. PLEASE read this and you will know what i am talking about http://www.tomshardware.com/reviews/memory-module-upgra...
and i would recommend you to make a separate loop for your video card if you want to overclock your CPU up to 4.2-4.4 GHz. gtx 590 produce a lot of heat + if you planing to overclock your GPU too. I hope my information was helpful