Speaking from experience, really.

After blowing 2 series 7 nVidia cards and 2 PSUs (one 500w and one 600w) I learned a lot about PSUs.
The wattage doesn't give a true reflection on how well it will perform.
The Crucial 600w PSU I blew was putting out 22Amps on the +12v rail (there should be a label on the PSU with a table showing all the ratings).
I now have a 500w that's putting out 35Amps on the +12v rail and I've never had any more problems.

Some good reading here: http://compreviews.about.com/od/cases/a/PSUWattage.htm.