it just depends on your clocking.
I run with low power costs, so I run my 7950s over 320w and over 640Mh/s each, and would tell you more like 1025watts at that level.
So, you have a nice range to work with there, depending on how much you want to overclock and what your electricity costs.
+1
I'm running 3 - 7950s (~625Mh/s ea) on a 1250w 80+ Gold Seasonic PSU and my wattage is hovering around 1050. Kind of makes me nervous about adding another card but I know those 3 are very overclocked (around 1150 & 1600) and could be pulled back in the event of another GPU.