Should I install a power fan in my attic?

Not only no, but definitely not, well unless... First things first - attics are meant to be hot, if you have a cool attic that means is it either really cold outside or the air conditioning is helping to cool that space. A secondary item to keep in mind is that air, water, etc... flows seek the path of least resistance As for the unless that only applies in one case & that is where you need more "upper" ventilation & you have enough "lower" ventilation to supply the air. Getting to your case - where would the fan be getting it's air from? Well there are 3 locations unless you have ductwork in the attic & then there are actually 4. Remembering that air will seek the path of least resistance the air for this fan would be coming from the ridge vents, duct work if up there, and then a toss up off air from inside your house & the soffits. In your case, get baffles put in every bay, open up the lower venting, air seal between the living space & attic & then fix / bump up your insulation to at least an R38/49 and maybe even 60 if you can swing it. Now it won't really matter how hot your attic is. Make sure if you do have ducts in the attic - that you air seal them (you can lose 50% of the HVAC efficiency & cooling ability) with only 10% leakage - Yes most of us would prefer them inside the envelope but if you cant do that, do what you can For more on Attic Ventilators: http://thehtrc.com/2011/rip-solar-powered-attic-ventilators