If you're dreaming of building a top quality system, with a dual-core processor, top of the line ATI CrossFire or Nvidia SLI graphics setup, and a bunch of hard drives, than you'll probably need one heck of a power supply to keep everything running smooth. Especially if you plan on overclocking the beast. There aren't many 700w+ Power Supply Units out there, but if you feel you need one, than Legion Hardware has a review you might be interested in.
"I began to look at power supplies and quickly realized there were few options available above 700 watts, at least ones that I would trust. For me power supplies are like memory modules, there are only a few select brands I would recommend touching. The majority of power supplies rated at 500 watts are never going to output nearly that much power and it gets worse the higher you go. The candidates that I dug up to power my gaming system included the Thermaltake ToughPower 750w, Silverstone ST75ZF 750w and of course the OCZ GameXstream 700w, which I am currently using."