I'd to add in case someone stumbles upon this thread.
The power supplies have different rails for required voltages. On the side of the power supply should be a sticker, on the box, or in the manual. It will be a chart that indicates the different rails and how many amps they can supply. There should be a section that indicates the max amp the 12v rail or rails can supply. This is the rail your GPU uses. Most power supplies if the total wattage is more than what your GPU requires, you're fine. But if you're paranoid about over working your power supply or worried about the stability of the card you can find the watt requirement for the card and find how much wattage the 12v rails supply. Remember 'volts x amp = watts'.