To use it you need SLI Mobo and 2x (or 4x) Identical GFX cards that support SLI. By Identical I mean same Chipset, same manifacturer, and same model, Unless you buy 2x Identical GFX from the start it's nearly impossible to go SLI, because it's hard to find Identical products later on.
You can have 2 of the same graphics cards from different brands, and run them in SLI.
1 EVGA 8800GTS 320MB + 1 XFX 8800GTS 320MB in SLI.
Different brands, same architecture, SLI.