Little update, I built 2 rigs (One for a friend and I). As I couldn't get any PCI-e risers I've stuck with 2 cards per board atm, once the PCI-e risers arrive I will raise the front support bar and separate the cards and be able to put more GPU's on each rig.
Parts lists for the 2 boards are;
Left Board
Gigabyte GA-970A-DS3P Socket AM3+
Sapphire TOXIC R9 280X x 2
Powercool X-Viper 950W Modular 80+ Bronze
4 GB 1333 Generic Ram
AMD Sempron 145 Single Core CPU
USB Stick for BAMT
Right Board
Asrock AM3+ 970 Extreme 4
Sapphire TOXIC R9 280X x 2
Seasonic 850W M12II PSU
4 GB 1333 Generic Ram
AMD Sempron 145 Single Core CPU
USB Stick for BAMT
Each rig gets about 1400kh/s but I haven't fiddled much with settings/config yet.