AMD's CrossFireX details leak out

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.