Advantages Of CrossFire Over SLI
Currently, in order to run a system in SLI, both of the graphics cards have to be identical, with the same BIOS. With even the slightest variation, you can - and most likely will - get errors. NVIDIA has yet to reveal whether limitation is driver or hardware related, but for the time being at least, the SLI solution can only be described as inflexible.
With CrossFire, current owners of X8xx series cards can upgrade to dual graphical solution by purchasing the CrossFire Edition motherboard and graphics card. If you own one 6800, you need to purchase the motherboard and the identical 6800, or flash the BIOSes to make them look identical to the system. CrossFire saves the X800 or X850 owner one purchase, because they can use what they already have.
With the ATI solution, the BIOS, physical makeup of the card, memory amounts, vendor origin, and almost any other variable is irrelevant - it only has to be close enough. Unlike SLI, you could say that "close only counts for horseshoes, hand grenades, and CrossFire."