I would choose Venice over Florence. Train to Rome. Take a day trip from Rome to Orvieto (pop. 21,000) which is not exactly a small town but different from the larger cities.

Everyone has their own interests - so it's hard to be specific without knowing why you have picked the places you have.

If it were me I would bag Milan (a business city unless you are heavily into fashion) and Lake Garda and do Venice, Florence, Rome and Amalfi Coast - which is already a lot to cover in the time you have.

We really have no big interest in Milian. We thought it would beeasier to fly into there maybe stay for a night and head to Lake Garda. We are considering Garda because of its beauty. I don't really know what much about it.

Igresham, I assume you've seen beautiful lakes before?

I would suggest:

Fly into Venice;
Nights 1,2,3 in Venice
Day 4 train to Florence early morning
Night 4, 5, 6 in Florence.
Day 7 morning train to Rome.
Nights 7, 8, 9, 10 in Rome.
Day 11 morning, train to Naples and then on, via regional train, to one of the Amalfi coastal cities.
Spend nights 11, 12, 13, 14 in that coastal city. You can bus between the small towns during the day if you wish. Better yet, might want to hire a driver for a day or two to do the coastal areas.
Day 15, train into Naples and spend the night there in preparation for your return flight home.

Buon viaggio! You will love Italy. It is a magical place!

What about Lake Como? Do you think I'd have enough time to visit there instead of Lake Garda?

Florence is nice, but not even close to my favorite place in Tuscany. The town of San Gimignano is nothing short of amazing. I also loved Siena. Monteriggioni is another cool town. If I were you, I'd find a south of Florence and spend a day or two doing day trips around Tuscany.

Also, I will second Igresham's recommendation of Como. I enjoy all of the cities of Italy, but I find the smaller ones more enjoyable. You get a better feel for the real Italy. Last time I was in Rome, I heard more people speaking English that Italian. Hopefully May won't be quite so crowded with tourists.

Second nytraveler and Sarge56's recommendations, although I might add a night or two to Florence and take it from Rome or the Amalfi Coast. Rome, Florence and Venice are the famous cities in Italy for a reason and the Amalfi coast is beautiful, but you don't need all that much time to see it.

As to what to see in Tuscany, if you're into art and architecture at all, you'll want to spend time in Florence rather than outside. Take a day trip to Siena, San Gimi, Lucca or Pisa.

If you're not into art and architecture, then three nites in Rome should be enough, plus the day you arrive from Florence.

Some years ago, we stayed in Sorrento. We loved it---rented a car and drove from Sorrento to Paestum and back (one day), took a day trip to Capri. Sorrento is a little more accessible than others in the area You should see Pompeii and Herculaneum while near Naples and if you do, then you will need the fourth nite. If you care nothing for archaelogy, then three nites should be enough. Best way to Pompeii and Herculaneum is by train (even if you have a car) as traffic around Napoli is bad even by Italian standards.

My husband and I flew into Venice (from Paris where we had spent the first 4 days of our trip) and stayed just that night and the next day. I am so glad we put Venice in our itinerary it is amazing, especially very late at night when everyone else is back at their hotels or cruise ships in bed. We walked the city, took the Vaporetto up the Grand Canal (as we had heard that the gondola rides were overpriced and over rated) went to St. Marks square it was quiet, beautiful and very romantic. ( from about 10:30 till almost 2 am) We used the next day to see other city sites and have a nice lunch along the canal. It is is full of tourists and cruise crowds during the day but still worth seeing. I feel a day or two max would be plenty for Venice. Two tips however, it is VERY easy to get turned around/ lost in this City so know where your hotel is especially if you are getting in at night. Also, buy a Vaporetto pass when you arrive (we came by bus from the airport) then know which Vaporetto line will get you closest to your hotel, then walk the rest of the way. You will save you a lot of money this way rather than taking water taxi's. We rented a car in Venice and drove to the Florence Tuscany area. Stayed in Tuscany first BEAUTIFUL!!! wish we would have spent more time here!! We stayed at the Villa where Mona Lisa lived and "Much Ado About Nothing " was filmed , amazing property in Chianti ! We drove from there to Florence for two more nights. ( rental return place was very hard to find, luckily a local helped us) we considered driving to Pisa and returning the car there but decided we didn't care that much about Pisa.
When in Florence, have the concierge at your hotel get you reservations for any museums you may want to see as these lines are truly out of control and reservations cut the waiting time at least in half. I enjoyed Florence it's a little dirty, but steeped in cool history, honestly the surrounding areas are where the beauty is at. We took the high speed train from Florence to Rome (well worth the extra money) and spent the last three days of our trip there. I would spend a couple of days in Rome for sure, so much to see and do.
We LOVED our trip. If I could change anything I would have chosen to spend a little more time in Tuscany exploring the countryside. We Traveled last Oct 2012. Weather was perfect.