I use CCBill to handle my billing. It was a pain at first, trying to get them to approve the site, but it all worked out. The only bad part is that in order to accept Visa, you need to put a deposit of like $750. JUST FOR VISA.
The secure script is uploaded to your FTP and you just plug it into the HTML of the pages you want secure. Once that is installed, it is pretty simple. Check out my site and you can see for yourself, all my galleries and members areas are password protected.
I just started my site. I am still learning and trying to decide the best way to start promoting my site.
Good luck.
www.painful-ecstasy.com