There is a chain of pizzerias called Fratelli la Bufala that has a location near campo dei fiori.
I have never eaten at any of their locations but I'm sure it's pretty good. There really are not any neapolitan pizzerias that I would go out of my way for in Rome.
You have to go to Pizzarium near the Vatican for the best pizza a taglio (pizza by the slice) I have ever had. Pizza a taglio is really what Roma is famous for as far as pizza goes. Their wood fired pizza is a lot thiner and cooked longer than a pizza napoletana. More crackery if you are into that kind of thing.
Pizzarium is located at 43 Via della Meloria and if I remember correctly is closed on Sundays.
Fratelli la Bufala is located at Via della Corda,3/5
Campo dei Fiori/P.zza Farnese
Telefono / phone: 06/6879001