If you wondered how CrossfireX works, now we have an answer for you. We asked several AMD product managers and they confirmed that the 3870 X2 dual RV670 card, or Crossfire with four 38x0 cards, works with the help of Alternate Frames Rendering magic.
Alternate frame rendering is quite a simple model, where the first chip renders the first frame, the second chip renders the second one, and so on.
If you have as many as four cards, each card will render two images, such as card one renders the first and forth while card two renders the second and fifth frame, the third card renders the third and sixth frame, and so on.
This should give AMD better performance and leaves space for an eight-way crossfire. In 3Dmark06 debugging mode we had a chance to see that four cards are rendering eight images virtually at the same time.