SLI is a feature that allows two NVIDIA cards to work as one providing increased performance. SLI was first supported in the 1.0-8174 driver, for a review read the Phoronix NVIDIA Linux SLI (1.0-8174) article.

To be able to use SLI (Scalable Link Interface) you will need two or more graphics card that are capable of running in SLI. You will also need a certified power supply that can handle two or more graphics cards. You will also need a motherboard that supports SLI, typically a motherboard with two PCI-express x16 lanes, and a graphics chipset that is capable of supporting dual GPU's.