An iPhone purchased in the USA can only be used with an AT&T SIM. You cannot place the SIM of another network in it. It will not work. The price of $199 for the 16GB iPhone 3G S and $299 for the 32GB version is with a 2 year contract with AT&T.
You will be better served purchasing the phone in Italy where it would be available to be used with a SIM from any network.
See the following page which identifies the different countries, whether the phone is locked to the carrier and whether it can be unlocked.
http://support.apple.com/kb/HT1937