Too many people under estimate the importance of a strong, quality power supply. Choose one from Corsair, Antec TP, or Seasonic...
I go a bit over on the size of power supplies, my goal is to keep the supply running dead silent (see below)
650watts will work for any single GPU system.
850Watts will work for all but the highest end dual GPU systems.http://www.newegg.com/Product/Product.aspx?Item=N82E16817371021http://www.newegg.com/Product/Product.aspx?Item=N82E16817139011
Here is a link to show how I determine if the supply will remain Silent under full gaming load....
I love the Corsair 850HX supplies because they remain dead silent to almost 600watts... http://www.corsair.com/power-supplies/modular-psus/professional-series-1/hx850w.html
You get silence to 525watts with the 750HX http://www.corsair.com/power-supplies/modular-psus/professional-series-1/hx750w.html
Here is a BIG difference the 750TX is Only silent to 375 watts!!!! (so, there is obviously more going on between these models than Just modular vs non-modular...) http://www.corsair.com/power-supplies/non-modular-psus/enthusiast-series-1/enthusiast-series-tx750-v2-80-plus-bronze-certified-750-watt-high-performance-power-supply.html
But THIS is what you need to watch out for , the 650HX is only silent to 260watts !! http://www.corsair.com/power-supplies/modular-psus/professional-series-1/hx650w.html