I recommend spending a little bit more time in Rome as well. Florence is charming, but I think you can see most of the highlights in 2 full days (you'll feel a lot more rushed in Rome if you only spend 2 days). Lines for the Uffizi and Academia are much less painful if you reserve ahead of time (whereas you HAVE to spend the time in line for the Vatican).

Oh and a quick rec: If you don't mind hostels, the place I stayed at in Florence, http://www.locandadaniel.it/, is absolutely charming and friendly and will make reservations for you at the museums. I met some very cool people there, too. Plus, the awesome view of the Duomo from their back balcony is real.