There are always loads of touts (baggarini) outside big games in Italy, so it's well worth a shot : I've always tried to leave it as late as possible to buy one, and you can end up with a reasonable deal.
Bound to be some aggro, but it's a bit like here - if you don't look and act the part, it's unlikely that anyone will start on you. But be careful !
went a few years ago to roma v inter and paid 10 euros on the morning of the game...massive game no better atmosphere than an italian game, shoud be fine for ticket massive ground !
Take your passport with you when you buy a ticket and when you go to the ground - they are supposed to cross-check the tickets with your ID. It's also worth having some proof of the fact that you are non-resident in Italy when you buy your ticket, because there are restrictions on who can get them - foreign non-residents, residents of the Province of Rome or holders of various football ID cards.
Prices range from €17 to €195 (although you can only buy the most expensive tickets from one of the AS Roma stores).
Tickets generally go on general sale three days before the game. More info for the Napoli game here:-
When I've bought Serie A tickets (not for a game in Rome, though), it's taken ages to complete the transaction and the queues have been very slow moving.