How to trick out your gaming PC with multiple graphics cards

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.

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.

Nvidia SLI and AMD CrossFire (seen here) multi-GPU configurations will work only on compatible motherboards that support the technology. Even if a motherboard has multiple PCI Express x16 slots, it may not be SLI- or CrossFire-compatible.

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.

Not all PCI Express x16 slots are created equal. Two slots on this Intel motherboard look similar, but one features a full 16 lane electrical connection while the other has only eight.

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.

With the exception of a few low-end products, Nvidia GeForce and AMD Radeon graphics cards running in multi-GPU configurations require a bridge connector to link the cards together. For more than two graphics cards, you must use larger 3-way or 4-way bridges, or multiple single bridges.

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 you install the necessary hardware and drivers, you must enable CrossFire and SLI in your graphics control panel. You can find CrossFire settings in the “AMD CrossFireX” section (above) under Performance in the Catalyst Control Center, and SLI settings in the “Configure SLI, Surround, PhysX” section (below) of Nvidia’s GeForce drives.

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.

A high-end, multi-GPU configuration will be able to run the vast majority of PC games at highest quality settings with smooth frame rates. In this screenshot, a pair of GeForce GTX 680 cards is able to consistently maintain a frame rate above 100 fps at a resolution of 2560 by 1600 with high-quality settings, FXAA, and tessellation enabled.

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.

However, multi-GPU configurations are not without their problems. First, multi-GPU systems will use more power and pump out more heat. They’re also more constrained by their drivers; for graphical performance to scale with the extra power afforded by an SLI or Crossfire setup, Nvidia and AMD must specifically optimize their drivers and include a profile for a particular application or game. Technically, every graphics card is only as good as its drivers, but SLI and CrossFire add another layer of complexity that needs to be addressed to realize any benefits from the additional graphics card or cards. Still, it's relatively easy and worth setting up if you have the space in your case (and room in your budget).

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