You shouldn't have to worry about sagging very much. I have the Zotac AMP! (non-extreme). As long as it's secured properly on the rear IO with the thumb screws it should be able to support itself fine in your case without issue.
Side note: the card will require two 6+2 PCI power connectors so make sure your PSU has enough connectors as well.
Well, I already ordered the Corsair Vengeance 650W. And from the popularity and quality of that PSU it must have all the connectors needed for modern hardware. I guess..