Crossfires take a very limited number of boards. There's like 3 to choose from, both mainboard
and video card.
SLI is supported by NF4 and above, but the mainboard has to have the slots. And you've got to have cards with the little SLI connection on top. If you look at your mainboard, you might see a small "card" insert between two of the long slots. Newer boards have the ability to forgo the physical switch for a BIOS switch.
The two long slots with a locking-tab at the end are PCIe 16x slots (although they are probably really 8x slots in SLI mode).
