SLI is only for PCI-Express. So no you can not "Scalable Link Interface" with another card, which by the way what's a 5600? Also, when you do SLI, you have to have to INDENTICLE cards, that means, same clock speeds, same brand, same everything. Although if you just go to http://www.newegg.com
and when you select the card you want to purchase just update the numbers to 2 of them, problem solved.