Nvidia have the right to the acronym SLI but don't get confused into
thinking this is Scan Line Interleaving,as it isn't.

From what i've seen only one card is responsible for generating the image,
the other is just a second GPU which performs tasks allocated by the first
GPU. This is far more efficeint than the Voodoo solution as no graphics
calculations are done twice.

It also means that cards don't have to match so another upgrade path
becomes available. You can add a new faster card as the master and keep the
old one as the slave GPU.