During the testing phase, I was able to successfully install two X1900 cards from ASUS and an ATI Crossfire Edition Master card. The RD580 chipset allows for the two video cards to work together in X16 mode on the PCI Express bus. Early ATI Crossfire motherboards only worked in X8 mode when two cards are used.
3/4 view of board
An ASUS X1800XT TOP card and a ATI Crossfire Edition card made up the other testing platform for Crossfire motherboards. Unfortunately, the NVIDIA SLI cards I meant to test Crossfire against have never materialized. NVIDIA says their platform has an advantage in games supported. My opinion is that if the cards aren't available, that is a moot point, as anyone wanting 7900GTs or GTX cards has a hard time procuring them today.