In Italy there are 3 GSM+W-CDMA operators (TIM, Vodafone, Wind) and 1 W-CDMA only (3) operator. There are also some virtual operators, but not so convenient. I'd go with a GSM operators (you need at least a dual-band 900/1800 phone; an American quad-band is perfect) It's very easy to get a prepaid sim-card, just go to a TIM, Vodafone or Wind shop and ask for a new "carta prepagata" (prepaid card).
Wind is the cheapest of the 3; coverage is a bit worse than TIM and Vodafone, but usually it's good (very good for US standards, I'd dare to say). I'd go with a Wind prepaid card with "Super Senza Scatto" plan.
When you buy the card ask the dealer to activate the "Call Your Country" option. With this you have cheaper tariffs to call Belarus (€ 0.20/min) and the US (€ 0.35/min). Activating the "Call Your Country" option is free of charge when you buy a new card, that's why you have to ask them to activate it at once.
BTW all information about Wind is here:
http://www.prepaidgsm.net/en/italia/wind.html
To refill it's very easy, most Tabacchi shops have electronic recharge: you choose the amount you want to recharge (i.e. 10, 25, 50€...) and tell them your phone number (32x.xxxxxxx) and they will recharge it automatically. Very easy.
In any case, when you activate a new Wind card, calling 4242 you can choose English for voice prompts and SMS communications. Ask the dealer to do it for you if you want, but it's really easy, just wait to hear something in English after a lot of info in Italian!
