What Are SLI and CrossFire?

A Detailed Explanation of Multi-GPU Solutions

A couple of years ago graphics manufacturer NVIDIA introduced a new technology that it called SLI (Scalable Link Interface), a technology that provided a computer with the ability to utilize two graphics cards to produce one higher quality video output.  Eventually ATI came up with their own such solution, which it called CrossFire.  Both SLI and CrossFire have the same basic purpose: to allow multiple graphics cards to operate in parallel, processing more graphics data in the same amount of time.  The benefits are obvious: frame rates in a game would go up, since more frames could be generated every second.  Alternatively, each frame could be a better image, but without slowing it down for the graphcs card to keep up.  You could also run at higher resolutions without a loss in frame rate.

How Does Each System Work?
SLI and CrossFire each achieve their purpose in different ways.  In an SLI system, the two graphics cards are identical and connected by a "bridge," which is just a high-bandwidth connection between the two cards so they can communicate and keep the overlap in processing to a minimum.  You plug your monitor into either video card, and the software takes care of the rest.  Effectively what you have in an SLI system is the equivalent of one video card with almost twice the performance, twice the memory, and twice the memory bandwidth.  The result: your graphics are rendered almost twice as well, whether it's double the frame rate or a higher level of anti-aliasing or anisotropic filtering (which each improve picture quality in different ways).

Very recently NVIDIA released drivers for quad SLI, which allows for four graphics cards in an SLI setup.  It works exactly the same way, though the bridge is a little more complicated.  For a while, quad SLI was only available through professional builders, and for good reason.  Four graphics cards, each using a lot of power, taking up a lot of space, and producing a lot of heat all in one confined space is a hard setup to implement in a system.