I recently went to Italy for the 4th time. It's a great country, and anyone who loves history should go. Rome and it's ruins are fascinating of course. Most travel articles and books will tell you not to get any tours and just go to the sights with a good history book and hang out at them.
Being hardheaded I disregarded this advice and got tours in Rome and Florence with Dark Rome tours. Some notes:
Vatican Tour: Great Tour, beautiful museum, and American tour guide with excellent English.
Colosseum and Roman Ruins tour: Bad English, bad knowledge of Roman History.
Florence tour: Bad English and bad sound equipment, but tour path was good.
For some reason Dark Rome didn't offer a tour of Venice, so we had to book through viator.com. Unfortunately the tour guide's English was poor, we couldn't hear her half of the time, and she got in a shouting match with a bunch of people in line at St. Mark's Basilica. That pretty much ruined the tour for us. While some might find an Italian Riot exhilarating we can see riots at any G-8 summit for free.
A movie to see before you go to Venice is Dangerous Beauty, which is the story of Veronica Franco, a famous Courtesan of Venice:
And, there is the impeccable miniseries Rome...seeing this before you go to Rome will enhance the whole trip: