We have several posts on the subject of travel from Rome to Florence and Florence to Rome by Train. If you don't find what your looking for in other posts I'd recommend you visit Rail Europe since they have many options and rail passes. Prices and schedules vary so I'd just check their site for the most current schedule and pricing options.
I found a really good deal on getting a multi city ticket. Its vaild for 2 months and another 2 months once you validate it.