If you are going for only one day then get there as early as possilble, I know it probably doesn't really need to be said but I am always amazed at those who said they couldn't get much done in one day then reveal they didn't get to the park until noon. In order for you to do as much as possible you will need as much time as possible. If you know your date then I agree purchase your Express Pass now along with your park tickets, these things are like the stock market the more in demand the more the cost. You will not need a date for your park ticket but you will to purchase the Express Pass. Pre-purchasing your tickets will also allow you to pickup your tickets at a kiosk rather than waiting in line, this will save you some time, you will just need the credit card you purchased the tickets with. If you are there for the day to hit the park(s) highlights an Express Pass will be of great help. Even on a slow day you can still have long waits on the more popular rides and and with Express Pass you should be able to do more than just the "big rides".
Though the Potter rides do not have an Express Pass entrance but they do have a single rider line. You will bypass almost all of the themeing of the que line but you will get on the rides in a much shorter time. Also something that is not mentioned much is that the Hogwarts Express does not have an Express Line either. Though the line for this is not usually long it can periodically have wait times over 30 minutes. The date you are going is a Halloween Horror night at Universal Studios so that park will be closing early. This will effect Islands of Adventure. I would think you will have many people transfering from Studios to Islands near closing and the line for Hogworts Express will be quite long. You will also want to do the Hogworts Express in each direction. It is a different experiece each way, though much better from Kings Cross to Hogwarts in my opinion.
One thing I try to do is travel the parks as light as possible. Attractions such as both Potter rides, the Mummy, Men In Black, The Hulk do not allow you to bring anything on, The Hulk can picky even about cellphones, hats and sunglasses. You must put your things in a locker. The lockers are free but in my opinion a hassle particularly at the Potter rides.
