We had a taste of NVIDIA's borderline-ridiculous option for performance enthusiasts a few months back. The results of this fearsome hardware configuration? A 33% increase in frame rates for Crysis on its higher settings. Sure, it's nice to play Crysis at its sexiest, but not for such a premium price.
Now Legit Reviews has a more in-depth look at the setup running Call of Duty 4, STALKER and Call of Juarez, all tested at 2560 x 1600.The Direct3D 10-based Call of Juarez saw the largest frame rate improvement of 39% over standard SLI, with STALKER and Call of Duty 4 both coming in at 15%. Essentially, the tests showed that a dual-SLI configuration is good enough in all but the most extreme cases. Perhaps driver and game engine optimisation would improve these numbers, but even then you'd be hard pressed to invest more in the GeForce 8 series with the 9 series just around the corner.
NVIDIA has additional info regarding requirements and compatible hardware at its enthusiast site SLI Zone. As you'd expect, a beefy power supply is going to be the cornerstone of keeping such a setup running.
Even with these new details, 3-way SLI still remains a novelty for the rich and professional overclockers. But it's still quite cool.
3-Way NVIDIA SLI Review - GeForce 8800 GTX x 3 [Legit Reviews]