Are you sure you have both the cards in the blue PCI-E slots. When I first setup my configuration I made the mistake of using the Crossfire X-Connect Cables that came with the video cards. These were shorter and meant I had to put one of my cards in a black slot. The M3A32 comes with its own crossfire cables that are longer than standard ones thus allowing you to place both cards in the blue slots.
Did you ever end up solving this issue? I've got a friend's computer that has this board and 2 hd4870s in crossfire, but they are only running at x16 and x8 respectively. I've been in contact with ASUS, but they can't seem to help and offered to warranty repair it. I need to know if it's something I can fix first though.