If one videocard can churn out 30 frames per second, two in the same machine should be able to pump 60fps, right? Well, not exactly. Assuming your PC is even capable of running more than one GPU at the same time, the best performance bump you can look forward to is about 80 percent in a dual-GPU configuration. Very high-end GPUs scale much less effectively.

The point is moot, of course, if your motherboard doesn’t support running two or more videocards simultaneously—and that means more than simply having a mobo with two or more PCI Express slots. Running multiple AMD ATI Radeon videocards, for instance, requires a CrossFire compatible motherboard. Doing the same with two GeForce cards requires an SLI-compatible motherboard (the acronym stands for scalable link interface).

Right: NVIDIA’s 9800GX2 in SLI mode is technically 4 GPUS on one board

It’s understandable that you can’t chain AMD and NVIDIA videocards together—the architectures are radically different—but there’s no good reason why you can’t mix and match videocards and motherboards. HP, in fact, recently figured out how to do just that with its Blackbird 002 gaming PC (which can be outfitted with two Radeon HD X2900 XT videocards in CrossFire on a motherboard with an NVIDIA SLI chipset). Unfortunately, HP isn’t sharing this firmware/driver trick with the rest of us.

Looking on the bright side, both companies support both AMD and Intel CPUs; gaining access to SLI, however, requires a motherboard with an NVIDIA chipset. CrossFire support is available with both AMD and Intel chipsets. Both companies’ technologies also require that the GPUs on each videocard be identical, although they don’t necessarily need to have the same clock speeds or even the same-size frame buffers. You can couple an NVIDIA GeForce 8800 GTS with a 640MB frame buffer to a GeForce 8800 GTS with a 320MB frame buffer, for instance, but you can’t pair either of those cards with a GeForce 8800 GTX.

TRIPLE AND QUAD GPUS

NVIDIA launched quad-SLI technology some time ago, but the solution failed to gain much traction in the market: It didn’t scale particularly well, it was wickedly expensive, and it was available only in pre-built systems from OEMs. The solution featured four GeForce 7900 GPUs mounted on four PCBs that fit into two PCI Express slots on the motherboard. NVIDIA never announced a similar solution for its 8-series products; and as we went to press, there were still no Vista drivers available for those rigs.

In the wake of Ageia shipping its PhysX physics accelerator last year, both AMD and NVIDIA made a great deal of noise about doing physics acceleration on the GPU. Despite several technology demos, in which a third videocard was used to accelerate physics, this initiative also failed to get off the ground. Now that NVIDIA has acquired Ageia and its technology, its next-generation cards may feature a built-in PhysX processor so you won’t have to buy a separate add-in card.

AMD recently announced CrossFireX technology, which will enable three and four videocards to operate in a single motherboard (one with three PCI Express slots, obviously), and NVIDIA was making noises about the same thing with SLI. As with NVIDIA’s quad SLI, all three (or four) GPUs will be used to produce graphics. NVIDIA’s new 780 and 790 nForce motherboards all support triple-SLI. With very large monitors becoming increasingly less expensive, gamers need all the graphics horsepower they can lay their hands on.

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