You need two card to enable SLI mode. They can be two cards of the same model, like a 7600 GT with another 7600 GT, but you CANNOT mix and match like ATI's CrossFire. However, improvements in the driver now allow you to use different brands when buying cards, but the models must remain the same before SLI is enabled. For example, you can have a eVGA 7900 GT and a BFG Tech 7900 GT and SLI will run perfectly fine. You just need the same model, but brands can be different.
Before SLI matured, users were required to have cards of the same model and from the same brand.