Making assumptions about # of drives, fans, and connected devices
Ya it might've helped if I included that. I'm planing on having one Samsung Spinpoint F3, a DVD burner, 1 200mm fan w/ LED, 3 120mm fans, and CPU cooler fan (120mm). Really no attached devices except maybe occasionally charging a MP3 player or something. I'd like to give it some room to breathe though, so I won't need to upgrade immediately if I add anything else. I'll probably end up going with the 850W.
It won't be at full load. And if it was, then surely 850W wouldn't have enough headroom itself to be desirable.
These reviews show the power consumption of systems with two 6970s being between 560-600W, varying depending on whether it's running Crysis or Furmark. The Legitreviews one explicitly states that they obtained this number from using a Kill-A-Watt meter, so that will be the power consumption at the wall. Therefore the actual system power consumption in that case would be lower due to the efficiency of the power supply they used.
Both of the above benchmarks used Nehalem processors: an OCed i7-920 and an i7-970, which are more power hungry than Sandy Bridge CPUs.
So based on the two reviews above, I think that 540W is a worst case scenario for the maximum power consumption of Priox's build. Which is ~72% of a 750W PSUs capacity. Which is admittedly a little high and why I wouldn't dissuade someone from buying an 850W PSU for such a build, if they didn't already have a power supply that was large enough to be safely used.
I think it also depends how often his computer is on full load. If it is not very frequently then there is less need to get a PSU larger than 750W.