840 core is nVidia's "OC mode" comparable to the switch on the 6990 but then through software. All will have to do 840 core even if the 590 will turn into a jet engine. I really don't get why they think about staying within the PCI-e specs in a moment like this. At stock mode the 6990 will win, what's the point of releasing this card then?
Well that's cool but I'd personally get 2 single cards.