Its all down to the chipset (bar that freaky MSI board, lol)
870 will do xfire if it has 2 PCI-E slots but the 2nd one will run at x4/x1 so isnt very effective
890GX does xfire at x8/x8 which is about 5% performance loss from full xfire
890FX does full xfire at x16/x16, each mobo should also state in its description, obv some boards do 3 way (tri fire etc)
each chipset has a finite number of lanes and as such distributes them accordingly
the 700 series is same as 800 series BTW