500W will do almost everyone. If you've got crossfire/sli 600W will keep you in "piece of mind" territory. 700W+ is for if you've got a hugely overclocked i7, crossfire/sli and a stack of drives, fans, ssds and expansion cards.
The OCZ are excellent. Very easy to use modular design and very quiet, they run cool too. I've got a 500W and a mate got the 600W on TWO for the same price, both are superb.