CrossFire: The Refresher Course
ATI's solution is similar to NVIDIA's SLI in three ways. The first and most obvious aspect is that you need two video cards. The second is that some means is needed to couple the cards together so they can act as one. Finally, just as SLI needs an SLI capable chipset, you need a motherboard with ATI's Xpress 200 chipset to make CrossFire work.
Above is the CrossFire Edition X850XT, and below, a standard X850XT. Notice the compositing chip underneath the clear section of cooler housing.
This is where the similarities end, however. ATI uses two different cards to implement CrossFire; the first is the CrossFire Edition of either the X800 or X850. The second card in the pair is one of the traditional versions of the X8xx series cards. The CrossFire Edition cards have a compositing chip that takes information from the secondary (slave) card and combines its pixel data together with that of the primary (master) card. Data transfer between the cards occurs using the special DVI cable that comes out of the secondary card and goes into the special DVI port on the primary card.