Charlie Demerjian, a ex The Inquirer editor who now works for Tom's Hardware, published some details about AMD's upcoming CrossFire X technology. He heard the scaling is pretty good but that CrossFire X support will likely be available for Windows Vista only.
On the more technical side, you can do 2/3/4 GPUs, and they do not have to be the same, although close is a good thing. The faster cards will more or less ramp down to the slower one but the memory speeds will not change. Memory disparity will cause the second card to wait every so often, so things are not running flat out all the time, but you should not notice any slowdown here. In any case, this is far from the end of the world.
What this means to you is you can put similar cards in the same box and get a speed gain. Similarly to Hybrid Crossfire, there becomes a point when the ramping down/waits will make the combo slower than the speedier card of the two alone, but as is the sign of any good architecture, it should work. You can off-road with your Ferrari Enzo if you feel the need, but don't expect good results. Look for the envelope of compatibility to expand with each new Cat release as well.
The most asked question is R680/3870X2 related, basically can you mix and match with those parts? The short answer is yes; internally the X2 is seen as 2x 3870s with slightly odd clocks. With a 3870 and a 3870X2, you will get 3x the clock rate of a 3870 and a little less than 3x the memory bandwidth of the trio.
CrossFire X support should be available next month with the ATI Catalyst 8.03 drivers.
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Re: AMD CrossFire X details leaked by Anonymous on Friday, February 15 2008 @ 14:18:42 CET
AND you can run dual monitors seemlessly under Crossfire...