A whole house fan is a good idea, but start by ventilating the attic, or making sure it is well ventilated. The cooler the attic, the cooler the upper floors will be.
A whole house fan can be incorporated into a forced air furnace, or can be installed in the attic space of the upper floors, and set on either a thermostat or a timer. I recommend a timer to encourage regular air exchanges, (avoiding "sick building" syndrome).
The whole house fan should duct to the OUTSIDE, not just into the attic cavity. Get some bids from some local HVAC contractors. Macdonald Miller is a good one, check the yellow pages under air conditioning or furnace.