Other methods like Tiling or Scissor, things get tricky, but those methods haven't been considered in years. AFR is by far the most reliable and is pretty much the defacto SLI method.
AFR, Alternate Frame Rendering
While one gpu is rendering the other is pre-rendering, off loading some of the work the primary gpu has to do.
The thing to remember is SLI was invented for AA/AS (pretty), not performance.
Although performance was a natural w/ the off loading of the work. It is usually about 1/3 or 30%.
So your favorite game w 1 video card = 100FPS add another video card and you will be aprox 130fps.
The first thing you need to know is what is the max amount of ram for your board. Ram is ram is doesn't matter if it video or system. You dont want to go over (including vram) the max. Doing so will slow your computer down.
Edited by DrFPS - 10/18/11 at 3:25pm