car or train?

Let's see:

A small car will cost about twenty-five bucks a day, that much again for insurance, that more again for parking. Oh, yeah, something for gas and tolls too.

Italy is great for cars, but not where you're going.

You might want to rethink a bit when you decide what you're going to be about in Tuscany.

The sequence is going to depend on your start and end points, which are . . . . .?

With only 8 days, you should limit yourself to 2 locations, 3 maximum. You won't want or need a car in Venice, Cinque Terre, or Rome. Driving around Tuscany you might need a car depending on where you plan to go there and what you plan to do. If you stay in Florence and want to see a bit of the countryside for a day, you can join a day long tour that visits wineries.

You could rent a car for a day in Tuscany, but waiting to retrieve it and then turning it in (not to mention finding places to park) will take up a little of your time, which is relatively short for trying to cover so much ground. Fast trains going a long distance (Florence or Venice to Rome) might be faster than driving those distances.

You should read the responses to this nearly identical recent posting:
Rental car vs. Train

For your first three destinations the car would just stay parked so it would be a waste of money to rent one. Tuscany, on the other hand, is great with a car. Public transport here is possible but time consuming...renting a car will allow you to see much, much more.
That said, 8 days isn't enough time for all four of these places...with only 8 days I would choose two. That will give you 3 days in each place...you'll need a day of travel in between to get from one place to the other. With your current itinerary you only have one or two days in each place at most. Pick two and save the other two for another trip. You don't want the memories of you trip to just be a blur!

Train service is frequent and affordable between the destinations you listed. For a start, see transportation connections summarized at the end of each chapter in Rick Steves' Italy book. We would only use a car for Tuscany, if you want to visit a few towns not served by train; for instance picking it up in La Spezia and dropping it in Orvieto or at Rome airport. There is also bus service in Tuscany, but schedules can be infrequent. See http://www.ricksteves.com/travel-tips/transportation/trains/italy-rail-passes for an overview of train ticket costs and http://www.ricksteves.com/travel-tips/transportation for more discussion of cars pro and con.

Thanks for the input! It was really helpful in making final decisions :-)