Unless i'm mistaken - for your uses you don't need a board that supports crossfire at all - just one with loads of PCIE lanes and enough space between them to fit four cards (with dual slot coolers). You then configure the cards individually and run miners on each of them. If you want to keep your costs down and only want your system to do mining then you could go for a 990FX board and a AM3 sempron or Athlon II CPU, however a £90
phenom II X4 955 would be a better bet imho as you can actually use this CPU to game when you aren't mining.
However, at this stage where the mining difficulty is really ramping up then I wouldn't suggest buying such an expensive setup purely for the use of bit coin mining. Have a look at
these graphs.