Rome - Siena - Florence

Between Rome Siena and Florence I would definitely do the train. It's the quickest and easiest way to travel between those places, as well as other large towns/cities in Italy (you'd also have to be downright insane to rent a car and drive in Rome!).

As for day trips around Tuscany, however, I would definitely rent a car if that's in your budget. The towns are fairly close together but it can take quite of bit of time on public transport to get around because of having to make multiple train or bus connections, etc. You'll definitely cover a lot more ground with a car, plus the driving is pleasant and non-stressful.

If that's what you do, I would pick up your car either outside of Rome (for example, in Orvieto) and visit various hilltowns enroute to Siena, then drop off your car when you get to Siena (you can't drive in the city center anyway). After you've seen Siena, catch the train to Florence.
Alternatively, you could catch the train to Siena, see it, then pick up your car on your way out of town, visit hilltowns, and drop off in either Lucca (nice town in its own right and worthy of a visit) or Florence.

The train might be easy, but is not the quickest way to travel between those cities. A direct bus goes from Roma Tiburtina to Siena (with the train you have to change at Chiusi Chianciano) and from Siena to Florence (some trains are direct, but some need you to change at Empoli; also, they are much slower than the bus).

Agree about renting a car around Siena.

Hi - Can anyone tell me what the best way to get from Rome to Siena is? I am thinking bus but if we rented a car and drove down is that easy enough to do? What website should i use for buses?

Thanks!