The cards must also be linked together using a bridge connector, which is usually included with either the cards or the motherboard, and the SLI/Crossfire feature must enabled in your graphics driver control panel as well (we'll show you how to do that in the next section).
Setting up your own Crossfire and SLI systems
Assuming you have plenty of cooling, a compatible motherboard, and a power supply powerful enough to power the extra graphics cards, installing those cards in a system for SLI or CrossFire is relatively straightforward; the process isn't much different than installing a single graphics card.
Begin by shutting down the system and unplugging it from the electrical outlet. Next, insert the graphics cards into the requisite PCI Express x16 slots on the motherboard and connect the necessary supplemental 6- or 8-pin power feeds for your particular cards. Then install the SLI or CrossFire bridge connector (or connectors) to link the cards together.
Once everything is properly seated and secured, connect your monitor (or monitors) to the primary graphics card--typically the card in the PCI Express x16 slot closest to the processor on the motherboard. Then power up the system, let your operating system boot, and install the latest drivers for the graphics cards.
After installing the graphics drivers, you may receive a notification that the system is SLI- or CrossFire-capable and be prompted to enable the feature. If not, simply open your graphics control by right-clicking on a blank section of your desktop and selecting either 'Catalyst Control Center' for AMD Radeon cards or 'Nvidia Control Panel' for GeForce cards from the menu, then navigate to the necessary menu to enable CrossFire or SLI. Find SLI-related settings can be found in the "Configure SLI, Surround, PhysX" section of Nvidia's GeForce drives, and CrossFire settings in the "AMD CrossFireX" menu in the Performance section of the Catalyst Control Center.
Bottom line: This is the pinnacle of PC graphics
What are the benefits of going to the trouble and expense of running an SLI or CrossFire multi-GPU setup? The most obvious benefit is increased performance; we ran a couple of quick tests and uncovered some huge performance gains after setting up Crossfire and SLI.
For example, a single GeForce GTX 680 card scored 3354 in 3DMark 11 using the benchmark's Extreme preset. That's a pretty decent score, but it jumped to 6463 after we added a second card and enabled SLI. We also ran the Metro 2033 benchmark with a single GeForce GTX 680 card, which put up 54.33 frames per second at high settings at a resolution of 2560 by 1600. Pairing two of the same cards running in SLI netted 93 fps, a fantastic score.
A couple of Radeon HD 7970 graphics cards showed similar performance improvement when using CrossFire. A single Radeon HD 7970 scored 3321 in 3DMark 11 and 58.67 fps in Metro 2033 using the same settings as the GeForce cards. With CrossFire enabled, the Radeons' 3DMark 11 and Metro 2033 scores jumped to 6413 and 99.33 fps, respectively.
In addition to increased gaming performance, having multiple graphics cards in a system gives users the ability to connect more monitors as well, or to leverage the GPUs for computing tasks such as the Folding@Home project or a number of other GPU-accelerated applications.