I have worked with integrating e-commerce web sites with several credit card payment processing services over the years (CyberCash was one of the earliest back in 1998-1999, and later CyberSource in 1999-2001), as well as Verisign's PayFlow Pro (2001).
More recently (2005), I integrated Paypal's payment processing service into a digital e-commerce web store application.
My earliest professional programming work involved developing a gateway processing service to interface regional bank ATM networks with national ATM networks and Visa's credit card processing servers.
Today I came across a useful article that summarizes some of the current options to consider for web site credit processing integration:
There are numerous challenges and risks associated with processing credit card transactions - and the level of attention-to-detail needed regarding security is not trivial.