SLI has been around for a few years and Nvidia have pretty much been the driving force behind multi-GPU solutions since the inception of their SLI solution. Last year they tried to ramp it up a notch with Quad SLI, but this failed to impress enthusiasts, review websites and pretty much the whole industry.
SLI stands for Scaleable Link Interface is the marketing name for a way of using two or more graphics processors in parallel. Using both the PCI Express bus and the proprietary SLI connector made by Nvidia, the graphics cards communicate using dedicated scaling logic in each GPU. Load balancing, pixel and display data are passed between each GPU over the PCI-e and SLI connector; basically the two cards share the workload.
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Review: OC3D