SLI and CrossFire are sister technologies that allow you to connect two graphics cards together, forming a kind of über-card. CrossFire (sometimes CrossFireX) is a feature on ATI cards, while SLI (Scalable Link Interface) is found on NVIDIA cards. The primary advantage of these technologies is allowing users to upgrade their GPU (graphics processing unit) and get latest-generation performance without paying latest-gen costs. Additionally, for high-end users, it allows you to get far more graphics power than is possible from just one card. So, there's a healthy number of reasons to upgrade via CrossFire or SLI.

This guide will assume a basic knowledge of computer hardware and installation, and mostly applies to gamers and others who need lots of graphical power in their computers.

Availability

Not all graphics cards and motherboards are equipped to support SLI and CrossFire. Supported graphics cards will have a small badge or icon on the box, or will note the compatibility in their product description. Generally speaking, "budget" or "value" graphics cards won't support SLI or CrossFire, and anything aimed at gaming or rendering will.

Motherboard support can be a little harder to determine. CrossFire is supported by almost all motherboards with Intel chipsets and two or more x16 PCI-E slots. SLI support is not as widespread, but includes all nForce chipsets and many Intel X58 and Intel P55-based motherboards. If there is any question, refer to your motherboard manual or manufacturer's website for confirmation.

Finally, to use CrossFire or SLI you have to have two graphics cards that are of the same or almost the same model. We'll go into the specifics later, but for now understand that cards connected in such a way have to be almost identical.

When to Use CrossFire/SLI

From the above section, you know whether your current setup (or the one you're building) will support one of these technologies. However, just because you can use Crossfire or SLI doesn't mean it's necessarily the best idea.

If you're upgrading an existing computer, this is usually the best time to take advantage of CrossFire/SLI. Since most computers being upgraded will only have one graphics card, there's room for the second. Most importantly, buying a second card like your old one is almost guaranteed to be less expensive than buying a newer model, and you may have even better performance. If your existing graphics card was made in the last 3 years or so and is still available for a reasonable price, it's probably good idea to utilize CrossFire/SLI.

If you're building a new computer, it's usually counter-productive to use CrossFire or SLI. If you buy one superior graphics card, you'll end up with about the same quality and still have room for expansion later. Of course if you have the budget, two top-of-the-line graphics cards will certainly maximize your potential. When building a new computer, though, it's usually a good idea to make sure your motherboard supports CrossFire, SLI, or both - it never hurts to future-proof!

When Not to Use CrossFire/SLI

There are some circumstances under which it's probably not a good idea to buy a second graphics card and use CrossFire or SLI. Obviously, this isn't an option with laptops. Similarly, if you have a Dell or some other kind of pre-built consumer computer, any kind of upgrade or modification may void your warranty. Please refer to any relevant materials and manuals before modifying your computer.

Even if you built your computer, it may not always be cost-effective to get a second graphics card instead of buying a newer, superior card. If your graphics card is older than 2 years or so, the cost of finding a duplicate will probably be disproportionately high compared to the added graphical power. Compare the cost of a newer graphics card to that of finding a duplicate for your older one - if the older card is too expensive or no longer available, simply upgrade with a single, newer card and leave CrossFire/SLI for the future.

CrossFire

CrossFire is widely supported by motherboards and is easy to implement; just make sure that your graphics cards are CrossFire ready (check the box or product description). Consult the installation manual that comes with your card for installation instructions, illustrations, and specific directions.

To CrossFire two ATI graphics cards, make sure they are of the same model. That is to say, if you have a Radeon HD 4870, make sure you CrossFire it with a second Radeon HD 4870. It doesn't matter if one was manufactured by Gigabyte and one by Sapphire, as long as they're both 4870s. So, there's some wiggle room on cards needing to be "identical" - as long as they're of the same model, they should CrossFire. If the cards have different clock speeds or amounts of memory, that won't be a problem - however, cards connected with CrossFire will run at the speed of the slower of the two cards.

ATI's current generation of cards can actually CrossFire with other cards in their subseries (e.g. 58XX cards vs 57XX cards). This means that a 5770 will CrossFire with a 5750 or another 5770, but not with, say, a 5850. Similarly, all 58XX cards will CrossFire with each other, but not with any 57XX cards.

Official CrossFire FAQ

SLI

SLI isn't as widely supported by motherboards, and must be used on one of the following chipsets: nForce 4, nForce 500, nForce 600, or nForce 700. Select Intel X58 motherboards are SLI-enabled. Consult the installation manual that comes with your card for installation instructions, illustrations, and specific directions.

To connect two NVIDIA cards with SLI, the cards must be of the same GPU series (e.g. 8600 vs. 8800) and GPU model name (e.g. GTS vs. GTX). The cards can differ in manufacturer, card model name (not to be confused with GPU model name), and clock speed. As with CrossFire, cards connected with SLI will run at the speed of the slower of the two cards.

Official SLI FAQ

Caveats

• Make sure your computer's power supply can handle the extra load of a second graphics card.
• In some cases, CrossFire/SLI don't improve performance. In extreme cases, it can actually decrease performance; this problem is inherent to multi-GPU systems and is called micro-stuttering. Not all users will experience this issue, and different programs react to it differently. Updating drivers may help, as the GPU-makers continue to try to solve this problem.
• Older cards may have supported earlier versions of SLI or CrossFire, but these earlier iterations of the technologies weren't as robust or even implemented in the same way. For older cards (before 2007 or so), you'll probably be better off with a direct upgrade.