Just to update, power meter thingy arrived. With both 390's pegged in Folding@Home I'm seeing 630W draw, if I chuck the CPU into the mix as well I'm seeing just shy of 700W draw.
So assuming that the PSU is 85% efficient as per its 80+ Silver rating, thats 595W being provided to the system out of the possible 894W my 750W unit can provide, split into 744W +12V and 150W 5V +3.3V.
Almost all of that will be 12V usage in a modern PC though but you could probably knock off another 20W being used by non 12v items.
Another 170W left in it before it even hits its rating. Could probably use a 650W PSU.
Also a little bit baffled by the PSU calculators...