You have gotten some great advice here, but I as a HUGE HP fan, I want to throw in a couple more things.
Be sure to spend some time in the parks in the dark (provided your trip is during a time when the parks are open in the dark). The areas have a much different feel and seeing the dragon breathe fire in the night sky is amazing!
While the wands are not necessary, by any means, if this is a one-time trip for you, you love HP and you can find the time, casting spells is quite fun. The wand selection show is cool, but is only a "must see" in my opinion, if you or someone you know is chosen for the show. And, of course, you don't know that until you get there, so it's a crap shoot. It is entertaining either way.
If you end up staying on site and do get EE, as strange as this will sound, I would avoid the park with EE UNLESS you can be at the front of the line. We went during heavy crowds in June/July and waited longer to ride FJ during EE than we did at any other time during our stay, presumably because everyone who had EE all went to that one ride at the same time. On the other hand, if you go to the park that does NOT have EE and can get in the front of the line, when the park opens, people will be scattering to various rides and you have a better shot of a shorter wait.
Lastly, try to take your time and really absorb the atmosphere. The HP areas in Universal are, in my opinion, the most amazingly themed lands at any theme park anywhere. There is an incredible attention to detail that you may miss if you are rushing through. My sister and I spent 6 hours in Diagon Alley alone on our trip and I am sure there are still things we missed.