What is SLI?
Was originally a technology developed by 3DFX, who was a Video Card company, one which makes there own chipsets (Voodoo series - which was the best at the time). They developed this technology SLI (Scan-Line Interleave) which allows people to put two Video Cards (Voodoo) in to there system and the cards would work together.
One card would have the top part of the screen and the other the bottom. This as you can understand would speed things up a lot in intensive Graphics programs, from CAD to the most intensive of all, 3D Games.
nVidia brought 3DFX after there cards where too costly for normal users and nVidia’s line was fast enough for nearly everyone.
Having brought 3DFX they got SLI, but for a long time didn’t use it. Only in the last couple of years with the development of “PCI Express 16x” which is (twice as “fast” as AGP 8x).
SLI now stands for “Scalable Link Interface” under nVidia. The new system uses two cards as before (6600 GT, 6800x, 7800x series) but now works out where to give which card what data, unlike before by splitting it in to two halves you get one most likely doing very little work for the top half(sometimes just sky) or the other way around in cases. So now it works out how to give both an equal load (most likely by 3D objects/models).
Quad SLI?
Just like nVidia’s SLI but with four cards! 7800 cards are costing each at around $750-1000.
SLI doesn’t double speed nVidia’s SLI gets about 40% (max) faster then just one card, so what would 4 cards do?