Since you have a phone that will work in Italy, it should be a three or four band phone. Under the US FCC rules, you have the right to have your phone unlocked (If it isn't already) - so you can buy a Italian SIM card for Italy and not pay the big charges that you would be paying otherwise. Putting a new SIM card in is very simple. Depending on the situation, you might have to pay some money for unlocking the phone.
Also, depending on the model of phone you have, you may already have gps reception built in, and there is software for many of the phones that allows you to have GPS with moving map (and sometimes even voice directions) on your phone.
It's worth looking into.
Otherwise, there are many inexpensive GPS units that work well in Italy, and you can get European maps for them (you should also be able to get European maps for the phone/GPS/Software combo).
I just changed to AT&T and purchased an unlocked Motorola because I can buy a Vodaphone SIM and use it in Sicily (the next time we get to go there). I'm looking into modding it for use with GPS.