Although CrossFire is theoretically independent of the motherboard chipset, ATI says that it only works with motherboards based on the Radeon Xpress 200 CrossFire. Like Nvidia, ATI's aim is to sell customers a motherboard of its own making along with its graphics cards whenever possible.

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http://graphics.tomshardware.com/gra...602/index.html

EDIT:
ATI's latest pronouncements put the Radeon X800 CrossFire Edition on a par with a X800 XL (R430) graphics card and not as originally reported with a X800 XT (R420). The dual performance of an X800 XT (slave) card can only be exploited with a connection to the Radeon X850 CrossFire Edition.