I am trying to decide whether to reserve a space on a train from Rome to Florence and back; and if so, how much for the standard vs the fast train. We have to coordinate a plane to Athens on the 1st of June.
Go to http://www.trenitalia.com/en/index.html you can book you trains there and the e-ticket will be emailed to you. If your trip is 60 or more days out try using dates that match the time frames of your trip (example if your trip is on 8 August enter a date of 9 May or 6 June) this will give you an idea of prices. I did this for planning and finally reserving for my trip this May. Have fun!
You want to be concerned with the time to get from rome whichever station you arrive at in rome, to, the rome airport you will use to get to athens. When you fly rome to athens is it fumincino or the other? And from which train station?
Unless you have extra time, paying for the high speed train is usually worth the cost. On the first trip we accidentally got on the wrong train in a rush and realized it too late. Our fast train wouldn't have taken the almost 4 hours the slow train did. Also, many of the slow trains do not have air conditioning... The fastest train is about 1.5 hours and others are just over 2 hours. Sometimes they have specials on the fast trains - the details are on the Promotions page of the Trenitalia website. If available, you have to select on the 'Tariffa' drop down box on the booking page.