there are lots of forum discussions (other forums then this one) that go into it. do a search on the cons of attic fans, but here is an example
Attic fan pros and cons [Archive] - DoItYourself.com Community Forums but the jist of it is that if an attic fan will cool your house, your house is not insulated (sealed) correctly.
best bet is to ensure you have good insulation. my house was not old, but i added 10 inches of blow in insulation. the builder did a horrible job there and the original owner must not have checked it out before moving in. i also added radiant barrier to my attic and that helped a ton. i had that in TX so that is why i did it here. that was about $100 to do, insulation was under $300 and i havent had an elec bill over $180 yet this summer in my 2700 sq ft house.