iIf you are dual video carding, got a few HDDs, a couple dvd rw cd drives etc and lets say 4 sticks of RAM then a 550W+ PSU should be more than enough.
Also, what CPU will you be using?
Fianlly, make sure that you have a 24 or 20+4 pin mobo cable on your PSU along with 8pin CPU cable, not 4.
If you are planning on making a ridiculous build, ie. intel i7, 24GB DDR3 RAM 3x Video card, 3x2TB HDD, blu ray, DVD rw, DVD reader, card reader, a load of USB devices then you will need 850W+, i would even go as far as to say 950+ .
I would look at the corsair range because all come with 20+4 pin mobo cable, 4+4 pin CPU cable so it will fit no matter what and they are very high quality and come in a variety of wattages, up to either 1000 or 1200W, can't remember which, but still too powerful for anything you would need