Fan probably not. If the existing attic fan isn't doing the trick, I'm not sure what another one will. Probably look at adding to your insulation around windows/doors first. This is probably the cheapest update.
Next will be updating your condenser fan, though it's probably best to wait until it blows since those easily run several thousand dollars, as the newer ones are likely a lot more efficient than the one you have from 2000.
Check other structural things that could be afffecting your home efficiency. Also, try switching all your lights to CFL bulbs, and maybe a few major bulbs in heavy use areas to LED (like kitchen table, living room, or other ones that you use almost every day for >1 hour).