The simplest way to make your PC games look better is to buy a better graphics card. If you already have the best graphics card money can buy (one of these, perhaps), the next step is to install a duplicate and make them work together. Most savvy PC users have a machine with a discrete graphics card, but adding a second or even third card and running them together can lead to a big performance boost in demanding PC games.
This job can also be a little tricky, though the process has gotten much easier in the past decade. Consumer-class multi-GPU graphics configurations date all the way back to 1998 when 3dfx debuted the Voodoo 2 and the accompanying SLI, or Scan-Line Interleave technology. In those days, when two Voodoo 2 cards were installed in a system and configured properly, each card would alternate rendering odd and even scan lines. The end result was typically a huge increase in performance, with frame rates nearly double that of a single Voodoo 2.
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Most modern GPUs from Nvidia and AMD can be paired up to work together as well, but they work very differently than the Voodoo 2 of old. Modern graphics cards running in multi-GPU configurations typically render alternate frames or split a frame horizontally (or into tiles), with each GPU working on a particular portion of the frame. But pairing multiple graphics cards together still can substantially increase performance.
The process is actually pretty simple, so let's get started!
First: What's the deal with Crossfire/SLI?
Here are two words you need to know when it comes to multi-GPU rigs: Crossfire and SLI. Nvidia calls its multi-GPU rendering software SLI (short for Scalable Link Interface), in homage to 3dfx, though it's nothing like the old SLI system that shipped with the Voodoo2. Nvidia in fact acquired 3dfx a while back, leveraging that well-known brand, even though the underlying technology is quite different. AMD calls its multi-GPU rendering system CrossFire, presumably because it sounds cool.
SLI and CrossFire are fundamentally very similar. Two, three, or four separate graphics cards from the same product family (three Nvidia GTX 580s, for example, or a Radeon HD 7850 and an HD 7870) can be linked together within a system to distribute the workload of rendering graphics and (ideally) to increase performance.
Both Nvidia GeForce and AMD Radeon graphics cards feature one or two small edge connectors, usually along the top and front portion of the printed circuit boards. If a card has only one of these edge connectors, only two cards can be paired together for CrossFire or SLI operation. Cards with two SLI or CrossFire edge connectors, however, can be set up in two-, three-, or four-card configurations. The more GPUs that share a 3D workload, the better.
Because SLI and CrossFire are features specifically designed to increase performance in games, the technologies are ideally suited to gamers looking for higher frame rates. Having multiple graphics cards in a system brings some other side benefits as well, but we'll get to those later.
What to know before you go upgrading
Here are a few things to consider should you want to upgrade a system with a multi-GPU SLI or CrossFire setup. First off, you need a motherboard that has the necessary PCI Express x16 slots and that is also compatible with either or both technologies. You'll also need a case that can physically accommodate and cool the graphics cards and a strong enough power supply to feed the cards with adequate power.