A Cross Between Master And Slave
The division into master and slave card could, at first consideration, be thought of as a disadvantage over SLI, as you do not have to cast a thought to which card is the "master" and which is the "slave" in NVIDIA's case. A closer look, however, reveals possible advantages that the ATI concept offers.
First, two absolutely identical graphics cards are needed for SLI. The combination of cards of various make is thus impossible for practical purposes. Even cards by the same manufacturer cannot generally be combined under SLI if they do not have precisely the same BIOS version. A later conversion of a PC to SLI through the purchase of a second graphics card is only possible to a degree with SLI and mostly requires a BIOS update for the second card.
Owners of a Radeon X800 or X850 graphics card meanwhile can effortlessly combine their model with a CrossFire model. Different clock speeds, BIOS versions, pipeline configurations and indeed the manufacturer can be accommodated. The only restriction is that the CrossFire model must match the other graphics card - in other words, it must be an X800 or X850 version.
Life is thus a lot more flexible with the CrossFire concept than with NVIDIA's SLI. The master card, based on a X800 XT PE or X850 XT PE configuration, controls the various card clock speeds and adapts to the slave card vis-à-vis pixel pipelines. If the latter runs with only 12 active pixel pipelines, the CrossFire card will also downshift to a 12-pipe operation.
This flexibility comes at a price, however - in every sense of the word. Owners of a "smaller" X800 or X850 will have to pay a hefty surcharge compared to their own card, even if they gain nothing from CrossFire operation because the standard Radeon model or slave card sets the pace. A comparably inexpensive multi-graphics card PC, as possible with SLI and GeForce 6600 GT, is not in the deal with CrossFire. You could argue equally convincingly that instead of buying two good-value eight-pipe graphics cards, one 16-pipe model will do just as well. That still ignores the point that you could upgrade at a later date to enhance PC performance. However, this option has its restrictions with NVIDIA's SLI as the card's vendor and BIOS must be identical.