Telephone number and 3G in Italy | Advice needed (Rome)

Hi everyone! Could you, please, advise on what sim-card it is better to buy to travel through Italy (Rome-San-Marino-Florence-Pisa-Riomaggiore-Genoa-San Remo)?
Could you also explain how it works (i.e. in Russia when you buy a telephone number in one city - in another the charges will be higher due to the roaming - is it the same in Italy?)
And when you buy a telephone number can you also opt in for 3G Internet? Is the connection good between the cities?

Many thanks!
And have a nice day! :)