Originally posted by: bamacre
A solid 350w should be fine. Hard drives don't use that much power.
Hard drives don't use much when everything is running and idle, however when you first boot up a single hard drive can suck up to 30+ Watt.
http://www.storagereview.com/WD7500AAKS.sr?page=0%2C5
Check that link, which basically compares power consumption for newer high capacity drives, other drives most likely will have similar power draw. Make sure you pay the utmost attention to the last chart.
To the OP, since you're planning to run 4 drives with an OC-ed processor, I would safely assume you will use extra fans and what have you. Therefore I would seriously advise using more than 350 Watt, just for the sake of safety. 350 Watt would seem to suffice (esp if you get ones from big brands, like PC&C, Enermax, and so forth), however I wouldn't be too comfortable having that PSU drawing close to it's max peak output.
Just a suggestion you might want to try this link that have PSU calculator (use the lite version, it's free):
http://www.extreme.outervision.com/