These aren't really "iPhone questions", per se ... and there's a lot of resources around to learn about international roaming and dialing, if you just search a bit. But one additional item you should look into:
Last time I was traveling to Italy, I was looking into getting a prepaid card locally. I found some consistent references online about needing to have some sort of Italian tax ID in order to make a purchase of a prepaid card. There was some Italian government website where I was able to get a tax ID as a foreigner (Stato Unati i.e. "United States") so that I could buy a card.
I never did end up buying a card on that trip - international roaming rates really weren't that bad (IMO) for the amount I was likely to use the phone. So I just kept my US SIM in the phone.
(nothing like getting a text in the middle of a romantic dinner in Florence "WHAT IS PSWD TO EXCHANGE SERVER? HARD CRASHED, RESTORING...")
So in addition to everything else, look into whether foreign guests are allowed to buy prepaid SIM cards directly, or if there is some other process you must complete first. I would imagine that an up-to-date travel book on Italy might have more info.
Enjoy your trip!