What they all said. We have both, and rarely run the AC. Keep in mid though, the attic/roof MUST have enough additional ventilation added to efficiently vent out the air that the fan will be pushing into the attic! A whole house fan moves a LOT of air, and it has to be able to get out or else you'll just pressurize the attic and kill the effect of the fan. Probably burn out the fan motor too.
This guy installed ours and the roof vents. http://www.colohomecool.com/?gclid=C...FQ4oaQodWqoAug