You can get an international driving permit from AAA. When I went two years ago, it cost me something like $15, but it's less if you're a Triple A member. I believe it is valid for one year. You can probably get by without it because hey, you only need a license if you get stopped.
One thing to be aware of is that some rental companies will not let you take a car into Italy if you rent it elsewhere, I guess because of a higher theft rate or something. The first place I tried in Germany (Enterprise?) would not permit me to take a car into Italy, period. The second place (Avis, I think) had a map that they went by: Western European countries were colored white and were OK to drive in, Eastern Europe was colored red and was therefore forbidden, and Italy was pink, which meant that you could take low-end cars but not expensive ones. I ended up renting a VW Polo TDI from them as it was the only car they had left. I would have loved to have rented an Alfa, but my only real stipulation was that it had to be something that wasn't available in the US. The Polo, incidentally, was a hoot that I got to about 110 on the Autobahn, and it got over 60 mpg.
Alfa or not, you'll have a blast. Enjoy!