Xfire and SLI may be the future of high grades PC...The GTX590 and 6990
are in fact Xfire and SLI on the same motherboard....
As a exemple,you can get today 2X560ti OC for $500.00 and get better performances than a GTX580 at the same price...And you can add a third one if you wish...
Anyway...Ruling out any technology from a brand new engine that is built for the future is a big risk...It may work, but it may also fail by comparison in the near future with more current and high-tech design...

Salute !