Could someone please explain the various lanes on entering & exiting the Autostradas.
I was thinking of using a credit card to pay.

Whenever you enter the Autostrada you'll pass through a CASELLO AUTOSTRADALE, where you will need to pick up a ticket which will show your starting point. When you get out of the autostrada you'll need to go through the casello again, this time in the exit direction and you'll see 3 signs: one showing TELEPASS (yellow), the other showing CARTE (blue), the other, an icon with banknotes (white). IF you have a credit card, you'll need to go through the gate under the blue sign. It's easy.
You can also get a telepass, which is a pre-paid card at the AUTOGRILL, a place on the autostrada where you can stop to eat, drink, and in some cases even sleep. WIth the telepass you pre-pay your access to the autostrada.

I forgot: the price you pay when you exit is calculated based on the starting point (point you took the ticket from, by entering the autostrada) and the exiting point. From Milan to ROme (about 600 km) you would pay about 30 euros.

Thanks BATUFFOLINA. I will be travelling from Fiumicino Airport to Poggibonsi exit. Is there a set charge per Kilometre ?

TO tell you the truth I don't know. From Fiumicino airport to Poggibonsi you're not going to be all the time on the autostrada. If I remember well, you'll need to take the A1 towards Firenze (north) (and you pay this section) and then drive towards Siena. It shouldn't be much money to pay, though.

My advice would be either to buy a good road map (De Agostini, Touring Club/TCI or also the Michelin one), or log on www.viamichelin.com and you'll get the directions. It's more or less 300 km (about 150 miles), it should take about 2 hours.

Actually, Telepass is not a pre-paid card. It is a little device that allows you to enter the Autostrada without stopping. The price will be charged automatically on your account.
You can rent it at Punto Blu office (close to the Casellos of the bigger towns), but you must have an account in italian bank.

You're right Marcolino, I was confusing it with the Via Card which is the prepaid card. Sorry for the misleading info.

Another query, are there any tolls on highways or only autostradas.

Only the autostradas are toll roads, the others (statale, provinciale, raccordo, tangenziale etc) are free.