Hopefully, your home was constructed with ventillation that met code at the time it was built, and the ventillation you want to add is to help keep the house cooler in the summer time. I like thermostatically controlled gable fans, but you want to choose a model that isn't really loud. In addition, when you install it, you will want to include a master power switch so you can turn it off when you want. Fans are rated at cubic feet per minute, and so you will have to figure the volume of your attic to estimate the number of air changes per hour you will achieve. This is all kind of pointless, because your heat load will vary depending on ambient temperature, prevailing winds, dewpoint, etc.
In other words, make sure your attic insulation isn't blocking any of your vents, and get a quiet fan that will help cool the attic when it really gets hot outside. In my opinion, figure on about 1 air change per two hours and that should be plenty, assuming you have adequate ventilation to move it through.