Bottom line is that you need a 500W power supply at minimum if your getting even a mid tier ati/nvidia card.
If the card takes up two slots because it's big and needs one to for ventilation purposes, then odds are you need atleast 500 W.
I still don't see the concern about power draw sometimes. Unless a card is pulling 30% more power than a similarly performing card, it's no big deal. You either need 500 W., 700-750W., or a 1000W. PSU. Usually when you're jumping to the next "threshold" of power draw, it's safe to say your going to jump 200W. in your PSU purchase.
And yes the purpose of their power draw readings (whether GPU only or entire system) is to give you an accurate idea of how much headroom you should have. PSUs can degrade up to 5% of their performance per year unless you spend the bucks on the premium brands and their premium models. Even out of the box they often do not output at their full marketed numbers, if you want that then you need to start looking for silver/gold ratings on the PSU.
Safe bet to have more power and not need it then to have too little power and need it.