It's no secret that it is tough to squeeze more performance out of Nvidia's quad-SLI systems in a usual game environment. But if you are running systems that ask for enormous pixel calculation, Nvidia refers to that as "extreme High-definition gaming," and use a fancy 30" display or Matrox TripleHead2Go hardware to run three displays in tandem, more graphics chips will deliver a better experience.
http://www.tgdaily.com/2006/08/09/diy_quad_sli/