Follow this link to the US Embassy and find out the documents you need to get married in Italy: http://www.usembassy.it/acs/marriage/general-marriage.asp
Is it legal? The wedding in Italy, for citizens from any country, will be recognized as a legal wedding in their home country. You will be supplied with duplicate copies of the legal wedding certificate, which you will use to register your wedding with your local city hall / county register / registry office upon returning home.
You will need also by law an officially recognized interpreter present at the civil wedding ceremony.