Slobogob :
Actually i'm not. While the chipsets you listed are crossfire compatible, they are not all 770 or 790 chipsets as the OP asked.
The 580x has, if i recall correctly, only two mechanical x16 slots but if used, both only use x8 lanes each (Same as the 790x). That's beside the point though.
To clarify it further:
The 770 is for single cards.
The 790X is for two cards in crossfire.
The 790FX is for xfire IF the board has the needed number of PCIe x16 slots. Some of the FX boards have only two.
The OP asked "is" a 790fx required....although probably hands down the best, it is not the only one.
All the chipsets listed will support (with a little more clarification) 2 cards CFX in full x16.
That, however does not mean that a particular board will have 1, 2 or 3 or more PCIe lanes, but that is the specifications of the chipset's capabilities.
Yes, the 580x, or rd580, or as more commonly known, the Radeon 3200 will indeed support 2 cards in crossfire at full x16 on both lanes, it was the first chipset to do so.
You may thinking of the RD570, which would support 1 lane at x16 and the other at only x8 in crossfire.
I am not sure what you mean "the 790fx is for xfire it the board has the needed number of slots" with "some FX boards only have 2" ??? All you need is 2 slots for Crossfire.