Each card can use anywhere from 200 to 240 watts. I would say to be on the safe side and not stress you PSU, I would be looking in the 700watt range to make sure it's will handle your system and both of the cards going at once and not run out of power. Always make sure you have more power then what you need. A PSU running at maximum is NOT running at it's most power efficent.
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I Feel Amazing Expendable! - Misc. Imperial Guard
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