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The big question today was whether or not we needed more than 16 lanes to feed multiple high-end graphics card in SLI, and the answer is a solid “perhaps not.”
OK, let’s call it a conditional "no.”
While we did see a fairly large difference between x8 and x16 slots when a single card was used, adding a second card shifted our limit to CPU performance. That is to say, for most of today’s tests, a faster CPU would be far more important than dual x16 slots in achieving the ultimate SLI performance.
That answer presents its own set of questions, since our high-flying Core i7 CPU was already pushed to 4.00 GHz. Most builders simply can’t go much higher with a daily-use gaming machine.
Another part of that conditional answer pertains to test resolution. GPU dependence increases with resolution, to the point that two cards eventually become a “bottleneck” far tighter than the CPU. Yet, that level of GPU dependence outweighs even PCIe x8 bottlenecks.
In the end, we simply needed a faster CPU to apply everything we learned about single-card bandwidth to multi-GPU configurations. This finding should come as some comfort to owners of “high-end” P55-based systems who might be considering an SLI upgrade for their GeForce GTX 480 graphics cards. If you have a high-performance processor in that motherboard, and you're overclocking to 4 GHz+, the extra CPU horsepower will have a more profound impact than an upgrade to an X58-based machine. Making X58 truly worthwhile requires an even faster processor and resolutions beyond 2560x1600.