One 8800GTX needed a lot less than 600W.
I suggested a 400W Forton in the sticky, though only a few other 400W PSUs would meet the requirements. The GameXStream would still be overkill for a 8800GTX, the 400W Forton would be at a safe 68% output at load.
As for your question about the connectors; that's a really good question actually, lol. I hadn't considered that and I'm trying to find out the answer as we speak. Since its an extremely recent problem, I'm trying to ask people that own Dual 8800GTXs (and there aren't that many of them to start off with). If your GameXStream only has 2 x 6-pin connectors, then I suppose that would be the end of it, since you would need 4 x 6-pin. The rails are nice, but if you don't have enough connectors....
Still trying to get a definitive answer though.