Personally, I'd skip the car altogether and ride the train. Italian trains are among the best in the world - regular, clean, fast, inexpensive, etc. And besides, you meet more locals that way and have time to look while riding. The train stations are usually in the center of the cities. Tickets can be obtained at the stations from either windows or machines. Credit cards or cash. From Rome, Florence is just a couple hours away and Venice maybe 4 more.

Romelle