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A brief look at three-way SLI


Insert sly French innuendo here
— 8:17 AM on February 18, 2008


A funny thing happened after our Radeon HD 3870 X2 review. I almost immediately transitioned over to working on our review of Skulltrail, Intel's eight-core monstrosity that we overclocked to 4GHz. Still woozy from Skulltrail's exhaust fumes, I then decided to turn my attention to yet another ridiculously expensive and powerful gizmo: three-way SLI.

At this point, my editor, if I had one, would probably be clobbering me. No doubt I could more profitably be spending my time reviewing hardware that most folks might actually, you know, want to purchase. And heck, I'm getting really close on that review of the 45nm Core 2 Duos—honest. But I couldn't resist a brief detour involving a howling phalanx of GeForce 8800 Ultras, mostly because I wanted to see whether we'd finally found the hardware equal to the task of making Crysis run smoothly at high resolutions and quality levels.

So what we have here is a quick foray into the warm and sordid world of three-way SLI. We'll take a look at how a trio of Ultras performs in a number of games, with a little extra emphasis on the system killer du jour.


Our three-way SLI test rig on the bench

What it takes to make a trio sing
Building a dual-GPU rig these days isn't exactly rocket science, let alone shotgun proteomics. You just have to select the right graphics cards, a compatible dual-slot motherboard, and a power supply that's up to the task. Generally, assembling the setup beyond that isn't any more difficult than building any other PC.

If you wish to add a third graphics card, though, matters become much more complicated in a hurry. Turns out that going up a weight class is no trivial—or inexpensive—matter.

First of all, Nvidia only makes two graphics cards capable of running in a three-way configuration, the GeForce 8800 GTX and its up-clocked brother, the 8800 Ultra. Both of these cards have dual SLI connectors onboard, and no other GeForce 8-series graphics card does. Since an 8800 Ultra can cost somewhere north of 650 bucks, investing in three is a substantial commitment.

Next, you're going to need a motherboard with three PCIe x16 slots and Nvidia's blessing. At present, that approval is limited to mobos based on the nForce 680i SLI and 780i SLI chipsets. You'll also need a fast Core 2 processor to populate that board's CPU socket. Nvidia recommends something in the 3GHz range with at least two cores, like the Core 2 Extreme X6800. Four cores would, of course, be even better.

Once you have the necessary cards in the necessary slots, you'll need to connect them all together, and for that, you'll need a custom SLI connector doober that looks like this:


Regular two-way SLI connectors ship with most SLI-ready motherboards, but this puppy is harder to come by. Nvidia has links to some places where you can order them online on its website. Prices appear to range between 10 and 20 bucks. That seems like an awfully lot for a connector doohickey, but hey, suck it up. If you let that extra expense put you off, you were never really a good candidate for three-way SLI anyhow.

Besides, there are much larger matters at hand, like the power supply question. When I was assembling our test system, I figured any one of the several thousand-watt beasts we have kicking around in Damage Labs would surely suffice. Turns out I was wrong. Nvidia recommends at least a 1100W PSU, and it will need to have a total of six 6-pin PCIe auxiliary power connectors onboard—two each for the three graphics cards.


This oughta do

In order to get the right connector count, we turned to this PC Power & Cooling Turbo-Cool 1200, the big brother of the model that delivered beautifully clean and efficient power in our recent PSU roundup. The Turbo-Cool 1200 handled our three-way SLI rig, no problem. Like its smaller counterpart, though, this PSU certainly makes its presence known with quite a bit of fan noise, even at idle. Such is the price of prodigious power.

Oh, yes. You're going to need a big, roomy enclosure to house all of this stuff and keep it cool. Nvidia recommends a Cooler Master Cosmos or a Silverstone TJ10. You may want to cut an extra hole or two in the side of the case and mount fans in them for additional cooling, as well.

Finally, you'll need to install Windows Vista on that new system in order to get three-way SLI working. Windows XP isn't supported. Nvidia also strongly recommends installing this Vista hotfix in order to ensure SLI performance scaling. That hotfix is one of five Vista updates they currently recommend.

How to harness a howling phalanx
What you get for your trouble, of course, is one of the most astoundingly powerful GPU configurations anywhere, with a total of 2.25GB of GDDR3 memory dedicated to video RAM and over 1.7 teraflops of raw shader power. In fact, don't even bother going for a three-way system unless you're going to hook it up to at least a 30" display with something like 2560x1600 resolution. Anything less would be a waste in most of today's games. You're going to want to push display resolutions and quality levels to the max in order to make the most of three-way SLI.

Those of you who remember Quad SLI will know why I say that. Quad SLI had difficulty delivering consistent performance gains over two-way SLI, especially in DirectX 9, where a three-buffer limitation hampered load-balancing efforts. Nvidia argues that three-way SLI avoids this limitation and is more broadly compatible, and they're probably right. Still, three-way SLI will almost surely need to be pushed in order to show its true potential.

For this quick look at three-way SLI, we've tested at a range of resolutions up to 2560x1600, starting with a focus on 16X anisotropic filtering and 4X antialiasing where possible. Rather than push on to extensive testing at higher AA levels, we then largely concentrated our efforts on Crysis, the one game that really seems to need the sort of graphical oomph a three-way SLI rig can provide.