crossfire is twice as good as sli, when you add another card you use about %70-%80 of the cards instead of Sli's inferior setup when you only get about %30-%40 of the second cards being used. Crossfire also has 3 different ways that the cards take the load.
see my other thread here http://www.computerforums.org/showthread.php?t=16092&highlight=crossfire
see my other thread here http://www.computerforums.org/showthread.php?t=16092&highlight=crossfire