Do you want to tame the beast? Seeking for 1-page-checkout or adding new tab? Magento’s checkout is one of the most complex area in the system, but following articles will place some light.
Adding custom credit card type to your payment method in Magento
At one point or another, you might want to implement a payment method with credit card types that are not implemented in Magento by default.
Please note that in this article you’ll learn how to add a new credit card type and validate it only. This article does not cover the whole payment module creation.