I have gone in August twice. You get extended hours at all the parks, but its very,very crowded. Definately stay on-site. You will need to take a break when the heat gets you. The way my daughter and I did it was......Early at all the parks...break around noon for lunch and a swim at the hotel (nap if you need to), then back to the parks for the afternoon parades. This is the hotest time of year and you will get ten min. showers here and there, but, hey! it's Disney and therfore its always worth it.:animwink:
Another suggestion might be, to join Disney Club to get some discounts on the hotels and admissions. It takes 2-3 week to get your membership packet (just joined) but it may save you some money. Discount airfare may be hard this late in the game, but again, you may get a discount through Disney if you join soon. Good luck and let us all know how you make out.