I've not been in August either (edit: I originally read your post wrong and thought you were going in the spring - oops!). If you would like to find out what the historic temps have been for the past several years for the week you're going, visit this site:
Weather Underground
There is a link on the main page to historic hi/lo temps for specific cities. It will give you a feel for what you can expect. I believe the low temps for August are around the mid 70's, so expect the mornings to start out warm with the 80's coming up fairly quickly.
As for EMH being crowded or not, there are two schools of thought (probably more than two, but I'm only going to cover two for you).
One is that the EMH is a great way to commando a bunch of rides right away, but since the park is typically more crowded that day as time wears on, it's a good idea to prepare to hop to another one when the masses descend.
The other is that EMH parks should be avoided altogether, and that you should visit a park on the day after it had EMH. This way you lose that extra hour, but you make up for that loss by having smaller crowds.
Of course, there's no infallable way to tour WDW. For example, the MK has EMH on Sunday, so if you follow rule #2 above, you'd visit it on Monday. But the MK is traditionally visited first by guests arriving over the weekend, so Monday can still be very crowded.
I guess I would say that your having to deal with large crowds would depend on these factors:
1) how flexible your itinerary is. If you find yourself surrounded by guests, are you willing and able to bop over to another park to see how things are there? Or are you married to your plan and wouldn't think of leaving?
2) how well you manage fastpass, and how well you take advantage of slower times of day for the big E-ticket rides. Do them first thing, do them last thing, and check them out during parades.
The better prepared you are the better off you'll be, crowd-wise. Remember that even on a busy day, it's possible to see and do the things you consider "must do" attractions.
I hope this has helped!
Jon