did you not you not see right under it it says min 500w psu thats just for one card so IMHO I wouldnt go under a 750watt psu but my personal recommendation is 850 for any sli or xfire setup even with the newer cards using less power the rest of your system and future expandability needs to be calculated in.
so basically I'm saying if ur not going to over clock anything and you have a very basic setup one hdd one odd a few case fans and the 2 cards it may work but it'll be at just over max recommended wattage. As theDTP you might be better going for the single higher card.