I've never used PayPal APIs, but I suppose APIs provided by a leader company like PayPal must work fine.
You may also check other payment services like Google Checkout for example. If you will use this way I subjest you to search the less expensive according you business
On credit card side (if you want to bypass Payment Gateways) I know that usually you must subscribe a bank related service (usually related your bank account) that manage all cradit card.
In this case interaction methods will be defined by bank and may change. I suppose that all banks provide SOAP WebServices because is a standard way to intaract.
Usually (in both cases) if you accept standard form for insert credit card data, instead use WebServices, you can add some html code provided by Payment Gateways or Banks that allow your site to accept payments.
In this case the only thing you have to do is create a style template containing html provided by Payment Gateways or Banks.
This article, for example, explains how to that on PayPal: How to Put an Order Form or Buy Now Button on Your Website Using PayPal
If you want to go deepen you can try to look here for more informations
Finally I subjest you to never save credit cards data otherwise you will have very hard security matters.
Feel free to ask for further doubts.