I'm building this rig:

MOBO gigabyte GA-970A-DS3P
CPU AMD Sempron 145 boxed
RAM 8gb DDR3 1333Mhz CL9 Geil
PSU 3x Fractal Design Integra R2 750w 80+ bronze
2x add2psu adapter
3x 1x to 16x PCI-E riser Powered
2x 16x to 16x PCI-E riser Powered
2x Aerocool external fan 120mm
1x Pendrive 8gb
5x Sapphire R9 280x Dual X cod. 11221-00-20g

Rig will work on BAMT 1.2 distro, controlled by my desktop via VNC

@crazyates: do you think that my experiment with 3 PSU (2 for 2 vga each, and 1 for 1 vga and mobo) should be good?
i have only a question about it, can the risers be powered by all the PSU or i must use only 1 psu for 5?