Yet more Crossfire news for you! From the front page:
ATI have just annouced that they now support Crossfire on the Intel 955XBK motherboard. Whilst we reported this earlier we have just found out more information, ATI are stating that in order to run an X800 Crossfire configuration you need an X800 Crossfire card, if you want to run an X850 Crossfire configuration you need an X850 Crossfire card.

This is a long way from what ATI stated would be the case with Crossfire, when mix-matched cards were originally one of the main selling points.
The word is that if your cards are in the same family they'll work together, and that looks to include All-In-Wonders and so on. However, the info on Crossfire seems to change a lot...