People have been able to run XFire 6950's on 650W PSU's however I would recommend at least 750W just to give you some headroom.
That PSU seems like junk, although it is 1000W it most likely won't output that much and if it does it will not be for a very long time. Stick to known brands that can actually output there advertised wattage such as Corsair, Antec, XFX, Seasonic, Delta and some others.
Also I realised that your motherboard only operates at 4x when 2 pci slots are active. I would suggest you find another board that operates at 8x each minimum to ensure better performance.