The whispers are that ATI is now asking mobo makers to make their high-end boards with slots for three 16x GPUs. All three slots may not need to have a full 16 lanes routed to them, 4 or 8 may work out fine, but the slot physically must be 16x.
If you are going to so Crossfire, at least two would need to be 16x. I wonder what they want the third card for? µ