I forget whawt SLI stands for but basically, it allows the use of 2 graphics processors to run on one system. People basically get 2 video cards and put them in the SLI slots and the motherboard will split the work between both computers making in theory, a graphics system 2x as powerful as their single counterparts.
SLI basically spills the work of one card to the second if the 1st card is overwhelmed.
Crossfire from Radeon is smarter, it actually splits the work between both cards.
Thank you very much. I know what SLI is now!
Scalable Link Interface
Two identical video cards work in tandem to process the graphics for the computer. The cards must be identical, there is a bridge or two that connect them so one card knows which portions of the page the other is doing.
It is kind of a workaround hack until the card makers can come up with a dual Graphics Processing Unit with Physics Processing Unit on one card.
It's only necessary when you are a hardcore gamer who wants to squeeze every last frame per second out of your machine.