I run a 750w Zalman.
It's better to buy a quality PSU that's correctly sized for your needs then to buy the biggest thing on the planet and never use it. If you only need 350w (peak) then get a 400w supply. If you need 500w (peak) then get a 600w. Don't buy a 1200w supply and only use 300w because it's much less efficient doing so.