Take the train. The European train system is fantastic. Consult Rick Steves guides for best information.

I think your best bet will be to get about by train. Especially between big cities like Rome, Florence, and Venice, connections are plentiful and very frequent to suit your schedule. Train fares are quite cheap, too. I've only taken trains between Bologna, Florence, and Venice, but found it very easy to navigate and quite a stress-free way to go. Have a good trip! Judging by my experience of traveling by car and minivan between Milan and Turin, I'd steer clear of the highways -- if if wasn't excruciatinly slow, it felt very chaotic at times!