If you already have a PayPal Business account you probably can already accept credit card payments through the Account Optional checkout. To test this for yourself, just add the email address from your PayPal account at the end of this URL:
If the first page that you see is just asking you to "log in", look for the "click here" link for users without an account and click on that. As long as the following page does not tell you that signing up is required for the payment, you are viewing the Account Optional checkout. (Most of your customers who do not have a PayPal account would see that Account Optional credit card page immediately, without seeing the "log in" page first.)
If you do not want to use the Account Optional checkout hosted on PayPal.com, you might be able to use Website Payments Pro to process credit cards via API. This is only available to US merchants and you must apply and be approved before you can use the Direct Payment API to process credit cards. If you do use the Direct Payment API, you must also offer Express Checkout on your website for customers who want to pay you from a PayPal account.