Express Pass will be helpful, we were able to get through everything we wanted to do in both parks by 7PM (starting with our 7AM early admission) this past Sunday (July 24th). We used Express Pass on all of the following except those that don't offer it (mentioned below) and the water rides as our pass was left in our locker. We did the water rides all at around 9-10 AM, and the lines were all 5 minutes each.
We were able to do the following, in one day in about 12 hours:
Forbidden Journey (x2) (No Express Pass) - Walk On
Flight of the Hippogriff - Walk On
Reign of Kong (No Express Pass) - 20 Minute Wait
Spiderman - 5 Minute Wait
Dr. Doom Fearfall - 5 Minute Wait
Ripsaw Falls - 5 Minute Wait
Bilge Rat Barges (x2) - 5 Minute Wait
Jurassic Park River Adventure - 5 Minute Wait
Dragon Challenge - 10 Minute Wait
Hogwarts Express to London and Back (No Express Pass) - 10 Minute wait on both ends
Lunch at Mythos
Walk to the studios
Escape from Gringotts (No Express Pass) - 45 Minute Wait
Shop in/Explore Diagon Alley for 2 hours
Men in Black: Alien Attack - 10 Minute Wait
E.T. Adventure - 10 Minute Wait
Transformers - 20 Minute Wait
Rip Ride Rocket - 15 Minute Wait
Despicable Me - 15 Minute Wait
Dinner at Hard Rock Café - Early Night back at the hotel.
We had planned on two days at Universal, but being the theme park commandos that we are, we knocked out everything we wanted (plus time for shopping for 2 hours in Diagon Alley) in a day, so got a full refund for our second day and went to Animal Kingdom instead on Monday.
But, like I said, we powered through everything extremely quickly, so if you prefer a (even a slightly more) relaxed pace, two days is perfect. But, don't be afraid to cross between the parks if you have a park to park ticket. Don't limit yourself on one park per day.