SLI - Scalable Link Interface.
This is the method used to 'link' two video cards (GPUs) together. This can sometimes be a drastic improvement in graphics performance, and games today are actually making Settings in the game specifically designed for SLI, such an example would be Call of Duty 2. The game has a setting directly for computers that use SLI.
If you want to enhance the performance of the SLI video cards, an SLI x16 motherboard is used. This improvment is not designed for higher end graphics cards, such as the 7 series GPUs. But rather it's targeted for the lower end video cards, liek the 6 series cards, mainly the 6600 models of video cards. This can drastically improve two 6600GT's in SLI, however, if you used higher end cards, you wouldn't really notice any difference at all.
The counter-part to SLI, made by NVIDIA, is CrossFire, which is made by ATI. CrossFire has a few advantages over SLI. One advantage is it's ability to use more 'modes' of the two GPUs. One such mode is SuperTiling. This basically gives a certain pixels, usually in squares, to one GPU, and the other to it's counter-part GPU. Think of the SuperTiling lay out to somethign similar like a chessboard, you know, black and white black and white. The same is applied to SuperTiling, one GPU gets the white squares, and the other gets the black squares.
ATI also has a x16 dual GPU format. This new speed increase is targeted towards the X1300 and X1600 series cards. Also, with the edition of the new ATI CrossFire 3200 chipset, you no longer need a 'Master' or 'CrossFire Edition' video card. Not to mention, it also makes the PCI-Express bandwidth a full x16, similar to that of NVIDIA's nForce 4 SLI x16 chipset.