When looking for a graphics card upgrade you may have come across these two terms, but just what are they? For the majority of buyers they probably won't influence which graphics card you purchase, but in a nut-shell they allow you to install a second graphics card (or third!) to work together with your first. The two graphics cards split the processing between them, improving performance. However it's not as simple as plugging in a second card and there are important considerations before going down this route.
Firstly, ATI graphics cards will only pair with another ATI branded card (CrossFire) and Nvidia graphics cards with another Nvidia branded card (SLI). Typically the two cards your pair should be the same model. You will also need a motherboard that supports the appropriate system, SLI or cross-fire, typically a motherboard will support one or the other. Check the Nvidia and ATI sites for supported hardware. Finally you will need a free PCI express slot in which to install the second graphics card. For the majority of pre-built PCs this may simply not be possible! If you are putting your own together then make sure you do some research first, the performance gains seen from adding a second card vary from game to game and are unlikely to ever reach 100% increase, some games may show no gain over a single card at all.
We'd say that for most people installing a new graphics card is the best upgrade they can make to improve gaming performance, but that they should stick to a single card for reliable gains. Why not take a look at our guide to choosing a graphics card as well as how to install a new graphics card? We feel CrossFire and SLI are best suited for enthusiasts willing to deal with some potential technical issues and those looking for the absolute best graphics performance.