I just ran across a fantastic explanation of all the moving parts involved in setting up credit card processing for online purchases. The guide is illustrated (helps a lot) and will explain in 10 minutes what will otherwise take you hours to figure out on your own with the unhelpful help files at most providers.
Go here: http://jumpstartcc.com/
While we’re at it, remember that the people who sell gateways and merchant accounts make their money off the setup charges and fees. You can negotiate. I remember some years ago being told I could set up an account for a $795 activation fee. I balked and asked if they could negotiate on that. They cut it to $250 in the next sentence. Yuck. (I found someone who offered the exact same service for only $95.)