Both
www.italotreno.it
www.trenitalia.it

but timetable & fares for May are not online yet. I guess they may be available at the end of 2012.

We are actually staying in Florence for 3 days and will be taking the train to Venice just for a long day trip. Thanks.
Barbara

The 2013 timetable will be loaded in mid-December 2012, but bookings usually open 45 days prior to the day of departure.

Thanks. Do you have a recommendation on which train line to use?

I am also going to be traveling next year by train between those 3 cities--April. Since the distances are short is it really necessary to be in FC? Is there a big difference between the classes both in price and seating? In order to have some flexability is it possible to purchase some sort of pass that could be used a multiple of times over a 10 day period?

We were looking at first class just because we will each have luggage we are traveling with and it looked like there was more storage space.

I am also interested in the difference between purchasing individual tickets and purchasing a rail pass and paying additional for the reservation.

Perhaps a bit more storage space - but you will also have many more US tourists w/lots of luggage traveling in First Class

Prices tend to fluctuate and railpasses can be nice - but probably won't save you significantly given your limited travel.

Trenitalia is the state owned rail system; Italo NTV is a new, privately operated competitor. I would wait until January and check to see if Italo NTV is still around - and then determine if their amenities/timetables make it worth your while.

Whatever you do, do NOT book through raileurope.com - as their surcharges for ticketing can sometimes double the price of the ticket.

Alice11 and bdemaria are giving you excellent answers, but perhaps you should do some more reading.

RinR maintains a phenomenal website, Ron in Rome, and his train articles might address some of your basic questions (however, the new Italo is not discussed):

Updated February 2011:
Buying Train Tickets – Updated

Updated March 2012:
Trenitalia and Booking Online, Using the Trenitalia Website – Updated

And the Slowtravel site maintains informational pages on train travel within Italy. Many of these articles are older, but for basic informational purposes they could be useful to you:
Trains in Italy

And in this ST thread there's currently one review of using Italo and some discussion comparing Italo with Trenitalia:
NTV (Italo): New Trenitalia competitor for high-speed rail

And I'm assuming you've already read this FTer thread which Alice11 linked above:
http://www.flyertalk.com/forum/italy...-operator.html