Double-barreled 3D graphics

The small row of pins along the top edge of this Nvidia GeForce 6600 GT card accepts Nvidia's SLI connector.

You may have heard some whispers over the past year about a computer using two 3D cards simultaneously. Madness, you say? We assure you, it's very real. Thanks to the new PCI Express interface, a properly equipped computer can use two graphics cards in tandem. It's not the first time we've seen this sort of thing; old schoolers may recall stringing together a pair of 3dfx Voodoo 2 cards back in the Windows 95 days. It is, however, the first time we've been able to pair two modern graphics cards. We haven't yet seen a complete, working dual 3D card setup in action (word is stability issues plagued the early parts supply), but it won't be long before twin 3D cards become as commonplace in high-performance PCs as laser-cut cases and liquid cooling.

How to double up
Right now, Nvidia provides the only way to run two PCIe graphics cards in the same system, with its scalable link interface, a.k.a. SLI. If you do remember the aforementioned 3dfx dual 3D card scheme, also called SLI (in that case, scan line interleave) you'll appreciate Nvidia's nod to the past. The new incarnation is currently available in prebuilt systems from PC vendors such as Alienware, Falcon Northwest, and Velocity Micro. SLI joins two matching Nvidia-based 6000-series graphics cards--in most cases, by way of a motherboard using Nvidia's AMD-only Nforce-4 SLI chipset and a pair of PCIe x16 slots. Nvidia also designed each of its SLI-compatible PCIe graphics cards with a row of pins along the top edge that can accept an Nvidia SLI connector that joins them together.

Nvidia's SLI connector bridges the gap between two 3D cards.

Your other option, of course, is to build an SLI system yourself. Nvidia has released SLI versions of some of its PCIe cards, including the GeForce 6800 Ultra, the GeForce 6800 GT, the GeForce 6800, and the GeForce 6600 GT. You'll need two of these cards to start. Then you'll need a motherboard that provides two x16 PCIe slots and the SLI connector to link the two cards. Asus announced its A8N-SLI Deluxe motherboard on November 17, 2004, and we expect it to show up in stores soon. Micro Star International has told us that its K8N Neo4 Platinum/SLI board will hit shelves at the end of December, as well.

According to Nvidia's Web site, you'll need two of the exact same cards in order to make SLI work. And not just two cards using the same chip--they must also be from the same Nvidia retail partner. For example, you wouldn't be able to join a Gainward GeForce 6800 GT with a PNY 6800 GT. The cards must both be of the same make and model. Power concerns are important because you can be sure that PCs running two GeForce 6800 Ultra cards will have monstrous power demands and require some intense cooling. You'll want to be sure to consult your motherboard manual before you purchase a power supply, especially because we found no such guidelines on Nvidia's Web site at press time.

This shot gives you an idea of what the inside of a dual 3D card rig might look like.

Other options
While the only option for a dual graphics card solution pins you to Nvidia and AMD for the moment, you can be sure that other players want in on the extreme 3D action. Intel and Nvidia recently signed a partnership that will allow Nforce chipsets to support Pentium CPUs, which means that Intel-based SLI motherboards can't be too far behind. If you're wondering what ATI is doing about all of this, you're not alone. We've asked, and the company isn't saying much. We don't expect it to be quiet for too long, though. You might also expect to see chipset makers such as Via make a foray into motherboards with dual PCI Express x16 slots. If these chipset makers can meet the standards of Nvidia's SLI certification program, you may see those motherboards include the SLI connector as the Nforce4-based motherboards do.

Questions abound
We have yet to see a working system with SLI or any dual 3D card solution, so we can only speculate about what performance benefits to really expect. Nvidia claims "as much as 2X the performance" as a single card; we've heard of up to 80 to 90 percent improvement over an equivalent single card configuration, depending on the application. We'll know once we can test a system or two, and we will report the results here shortly. We're particularly interested to see the performance difference between a dual-card configuration, for example two 6600 GTs, and a single, more advanced card that costs the same as the two cheaper ones, in this case, the 6800 GT.