http://www.techreport.com/articles.x/14161/1

Conclusions What can I say? The picture isn't terribly rosy. In terms of performance, what we saw out of three-way SLI was more […]
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Conclusions
What can I say? The picture isn't terribly rosy. In terms of performance, what we saw out of three-way SLI was more or less what we expected: that you'll need a 2560x1600 display in order to really realize the benefits of three GPUs. Even then, in none of the games we tested did we see a case where going from two cards to three made a real difference in terms of playability. (Going from 80 FPS to 95 FPS doesn't count.) The closest we came to that may have been Crysis, but that game seems to have its own performance scaling problems—at least at the "high" and "very high" settings—that involve factors other than the graphics subsystem alone. Perhaps, in some games, turning up antialiasing levels to 8X or 16X will help tease out some of the benefits of having a third GPU. That wasn't the case when we tried it in Half-Life 2: Episode Two, however.

Subjectively, playing games on a three-way rig wasn't especially exciting, either. The main difference I noticed was the noise emanating from the system, a reality we quantified with our power, noise, and temperature measurements. Three-way SLI truly is an extreme solution, and it comes with all of the drawbacks that such things tend to have. I'm sure it's possible with careful component selection, artful thermal design, and probably some form of water cooling to make a three-way SLI system that isn't especially loud. But it will draw lots of power and expend it into the room as heat, almost inescapably.

Personally, I don't think it's worth bothering, and I haven't even mentioned the expense involved yet. Two GeForce 8800 GTs should be up to the task of driving a 30" wide-aspect display reasonably well in most games, and a pair of those costs less than a single 8800 Ultra. If that's not enough for you, in my view, the very best config Nvidia has to offer right now is a pair of GeForce 8800 GTS 512s. This card is, like the 8800 GT, based on the new G92 GPU. As a result, the GTS 512 has H.264 decode acceleration for HD video playback, and it has more texture filtering and shader arithmetic power than the 8800 Ultra.

Having said all of that, I'm still glad that three-way SLI exists, simply because it expands the horizons a little bit. In fact, I'd like to see more-than-two-way setups with G92 GPUs become available soon, since they'd be more attractive overall. So long as a system like this is possible, somebody out there is going to be looking for ways to take full advantage of it. Crytek seems to be close. If and when they get there, it should be a glorious sight to behold.