Found a couple web sites on visiting Pompeii. It looks like you can buy what they call a Intact: € 20.00 ticket that allows you to visit 5 sites. Pompeii,Herculaneum and Oplontis, Stabia, Antiquarium of Boscoreale - validity 3 days.
The Herculaneum and Pompeii tickets separately are 11 Euro each. So even if you don't visit the other 3 sites, looks like you will save a couple Euro buying the intact ticket for 20 Euro.
Here's the web sites.
I know the Circumvesuviana train stops at Pompeii and Herculaneum. Not sure about the other 3 sites.
Are credit cards now taken for ticket purchase? Can they be bought in advance on-line?Edited: 7:04 am, April 02, 2014