You should expect heavy crowds and long lines during the first two weeks of August. It starts to taper off as the month goes on (as more and more kids go back to school).
Islands of Adventure can be done in one day **IF** you arrive early (before the park opens). Don't stop to do too much...just the rides. I usually get the rides done first thing, then do a second pass around the park to take in the shops, shows, etc.
The Disney parks are going to packed all summer. The Fairy Tale Package was a big success and a lot more people are going to be at the parks than they had anticipated a few months ago, so please keep that in mind. Again, arriving early, and touring intelligently is your best defense against long lines. Try and get the popular attractions done early in the morning, and be realistic. It's Florida, and it's August, so it's going to be very hot. I don't know about you, but my enjoyment factor tanks when I'm running around in 96 degree heat and 90% humidity.
Pete