HSR,
With such a short time frame, I'd suggest focusing on Rome and north this trip and cover Naples and Sicily on a future trip. Planning to visit 6 locations within 14 days is a bit "ambitious", to say the least, and doesn't allow for travel times. That only provides about 2.3 days in each city. Getting to and from Sicily especially will require significant travel time (even with budget flights, each way will take at least half a day).
Another thing to keep in mind is that you'll lose the first day in flight times and time zone changes and the last day will be spent on the trip home, so you'll only have 12 days for touring.
Under the same circumstances, I'd structure somewhat like this:
Day 1: Flight to Venice / Rome
Day 2: Arrive (recover from jet lag will take a day or two - check the Guidebooks for options on getting from the airport to Venice).
Day 3,4: Venice
Day 5,6: Milan (trip from Venice is only a few hours by train - be sure to watch your belongings carefully in Milano Centrale).
Day 7-9: Florence (trip from Milan is only a few hours). You could take day trips to Siena, Pisa or other nearby locations while in Florence - if you're considering day trips, you might drop one day from Milan and add to Florence.
Day 10-13: Rome (there's a LOT to see and do in Rome, so it's worthwhile to allocate more time - again, watch your belongings carefully in Roma Termini, on the Metro and especially on the #64 Bus, should you happen to use that - wear a Money Belt!).
Day 14: Flight home
If this is your first trip to Europe, Europe Through The Back Door will provide lots of information on how to travel "well" in Europe (especially the chapter on "rail skills"). The Italy 2010 Guidebook has lots of great information on Hotels, restaurants, Museum costs and hours, etc. and tips on things like avoiding queues. I find the Italy book is a good resource to pack along on the trip.
Happy travels!