For example, if your current PC packs an and you're looking to upgrade, you could shell out $500 for the more powerful Nvidia GeForce GTX 580. Or, you could spend $250 for a second 560 Ti and run the two matching cards in SLI mode.
SLI (Scalable Link Interface) is Nvidia's proprietary method of parallel graphical processing. It allows two graphic cards to share the work of rendering an image.To figure out which configuration was faster, we tested them both in the PCWorld Lab on the , on Codemasters' Dirt 2, and Ubisoft's Far Cry 2.
According to our lab tests running at 2560-by-1600 screen resolution on high graphic settings, the two pair of GTX 560 Ti cards running in SLI will spit out more frames per second than a solitary GTX 580 at the same resolution (113 vs. 77, respectively.)
Running off-road rally racer on the same test machine gave us similar results: with two GeForce 560 Ti cards running in SLI mode and all graphic settings turned to high we managed to eke out 115 frames per second while playing at 2560-by-1600 resolution, while running the same demo with a single GTX 580 card barely gave us 84 frames per second. In short, there was a noticeable performance improvement using two cheaper graphics cards instead of a single, more expensive card. The total cost of the two GPU setups is the same (2 x $250 vs. 1 x $500), but if you're upgrading from a single card it's much cheaper to simply double down, and you'll get better performance to boot.
Our lab tests suggest you can score a similar performance increase with Radeon cards by delaying your upgrade in favor of doubling up your current card. While upgrading the GPU in our test machine from an AMD Radeon HD 5870 to an nearly doubled our framerate running Far Cry 2, we observed an even greater improvement slotting in a second 5870 card and running the two identical cards in tandem using CrossFire. CrossFire is AMD's proprietary parallel processing technology for graphical renderers that (just like Nvidia's SLI tech) allows two cards to share the work of one.