Recently made an ultra budget build with an i7 920 2,6ghz and a 6950, got an EVGA 500W Bronze 80+ with it but considering how much the 920 draws when overclocked i'm concerned. It has 480W on the 12V and i'm not really looking into overclocking the 6950. Would it be possible to get the 920 to 3.5 or even higher? My motherboard is pretty much perfect and has an 8 pin + several heatsinks on VRM's and good OC'ing options.
You should be fine up to 3.66Ghz. Find a sweet spot where you can get good performance without adding too much voltage. source : http://www.tomshardware.com/reviews/overclock-core-i7,2268-10.html
Try to stay at least 20% below max power to reduce heat/noise, reduce capacitor ageing and allow your machine to remain stable for longer. Get a mains power meter and keep below 500W, preferably a lot lower. (a 500W 80% efficient PSU will use 600W when supplying 500W to the PC) Pushing a PSU hard will make it fail a lot earlier. I always buy a PSU that can supply double the PCs basic needs to give a large headroom for overclocking and expansion.