Train from Roma to Firenze

There are many, many trains that make that route. You can choose between the faster trains or the milk run trains depending on what your priorities are.

Take the Express for about 35 E

Like Mike says, lots of options.

If you're in a hurry take the fast Eurostar Italia--about 1.5 hours and 50+ euro.

If cost is more important to you, take the slower trains which I believe take about 3 hours and costs closer to 15-20 euro.

I'm afraid I don't quite understand your question about the rental car. You don't want a car in Florence unless you like traffic, no parking, and hundreds or thousands of dollars in fines for violating the ZTLs (zones that you aren't allowed to drive into). Discussions on how/where/whether to get a rental car in Florence to explore Tuscany are frequent on this board. If you do a google search (see the sticky on top of this forum) you will find more than you wanted to know. :-)

There are 55 trains a day between Firenze and Rome. Pick one you like.

Here are travel times and 2nd class fares.

EuroStar Italia Alta Velocita trains: 1hr25min for 44 EUR.

InterCity trains: 3hr15min for 28 EUR.

Regionale trains: 4hr4min for 16.10 EUR.