By connecting two graphics cards in the same computer you get a theoretic possibility to double your graphical performance. In practics you are never really close to this though. Both NVIDIA and ATI use an external bridge to link the data from the two graphics cards. NVIDIA has its SLI bridge which looks like a small circuit board and is fastened on top of the graphics cards inside the case. ATI in turn uses an external cable, called dongle, which connects the graphics cards in a Crossfire configuration.
Both NVIDIA and ATI’s multi graphics card solutions can be used without these bridges but then all of the communication is done through the PCI Express bus, which is already quite busy. At Hexus they decided to investigate how much performance one would lose if you decide not to use the SLI bridge or ATI dongle. The loss varies from a few percent up to almost 40 in some cases, which clearly shows that these bridges are necessary with high performance cards.
