I did WDW for a 9-10 day trip that included Memorial Day in 2005. Yes, it was crowded. However, it wasn't that bad. Not all schools in the USA are out for Summer vacation yet. I think the later you get into the Summer, the worse it gets because more schools let out. We had a great time! That was the trip that on Memorial Day itself we got picked to be in a parade.
I would think 4th of July would be more crowded than Memorial Day, but I have no way of knowing that for sure.