Italy: Buying Tickets in Advance
 

I want to purchase as many tickets in advance as possible for my trip to Italy from 9/20 to 10/11.

Therefore, any feedback would be great on which sites it is necessary to buy advanced tickets:

Venice:
Peggy Guggenheim Collection

Florence:
Duomo
Galleria dell'Accademia (DAVID)
Uffizi
Santa Croce
Palazzo Pitti
Boboli Gardens

Naples
Palazzo Reale (palace)
Catsel Nuovo (castle)
Museo Arch. De Nazionale
Museo di Capodimonte (museum)

Capri
Blue Grotto

Rome:
Too many to list, what should I book in advance?

Florence: we were there the same time roughly that you will be.
The Duomo does not require tickets unless you are talking about going up in the tower or something. It is a church.
You should buy advance tickets for the Accademia and Uffizi. You might buy advance tickets for the Pitti. You should not buy advance tickets for the Boboli Gardens. It is not crowded.

Venice: we didn't have any trouble getting tickets at the Guggenheim, but we were there a little later in October. Be aware that it is going to be very hot and crowded.

It shouldn't be terribly hot and crowded in late September and early October although you will always find crowds in the more popular destinations in Italy.

In Rome,if you plan to visit the Borghese Museum, get tickets in advance. Also for the Scavi tour and the Vatican Museum if you are interested. And, as noted, for the Accademia and the Uffizi in Florence. We normally don't get a lot of advance tickets traveling in late September.

The official website for the museums in Florence:

http://www.polomuseale.firenze.it/en/orari/

I don't think you need to buy Santa Croce in advance. I've never had to wait more than a few minutes there.

Naples - I would look at one of the Campania Arte Cards, depending on what you plan to visit. The 3 day Tutta La Regione card includes transport all the way to Paestum (27€).
http://www.campaniartecard.it/

Reservations are mandatory at the Borghese. If you plan to buy the Roma Pass, you have to call the Borghese and tell them you are using the pass as you can't reserve online unless you are buying a ticket. Best use of Roma Pass is the Borghese and Colosseum/Forum/Palantine (counts as one entry) for the two free admissions.
http://www.romapass.it/?l=en