To run 1,300 watts at 80% efficiency you'd need a 1625 watt power supply. I suppose you might find one somewhere, but it's bound to be expensive. I would go with (2) good 850's or higher.

5850 cards average 171 watts, so that's 1026W continuous just for the GPU. Add in 40 watts for CPU, some for RAM and cooling, etc... I think that's pushing a 1,200 watt supply too hard. I wouldn't doubt that it can be done though.

If I were going with a single supply and (6) cards I'd probably get the 1,500 watt Silverstone supply then. That should do nicely. Or you can get an adapter cable from cablesaurus that joins two power supplies on 1 mobo. OK, sorry I keep editing.