You should also note that there's a new bus (or coach as you might say) service directly from FCO to Naples (also to Florence):
http://www.fiumicinoexpress.com/?lang=en . For you, it would leave FCO at 16.20, arriving Naples at 19.07. It has its own complications if you book in advance and miss the coach; you could decide on arrival whether it's worth the wait at FCO to have a direct ride, with the bags in the hold.