I've seen alot of threads asking questions about SLi, soI decided to write an article (sticky please) to put an end to the hundreds of threads about the same question.
So, what is SLi? SLi is the linking of two graphic cards (GPU) through an SLi bridge. An SLi bridge is a small piece of PCB that connects the two cards together. The bridge can be found with your motherboard accessories. You will obviously need 2 GPUs, but, they need to be exactly identical for this to work. Also, they will need to be nVidia cards, (for ATi cards, see "Crossfire") and your motherboard will need to have 2 or more PCI-express x16 or 2.0 slots, and should be labeled "SLi Compatible." To enable SLi, you will need to install the two cards, then put the SLi bridge over the two small gold feet (approximately half an inch in length, and is found on the outer edge of the GPU.) You will also need to enable "dual GPU" with in your driver utilities software.
SLi gives you the performance of the first card, plus 40% performance of the other card. This means that you get a 40% increase with a second card in SLi. Also note, that while in SLi, you can only use the outputs of the primary card. That means if that for some reason you have more then 2 monitors, you won't be able to use SLi.
For more info straight from nVidia, click here:
Enjoy, and I hope this answers most of the basic questions out there about SLi.