IMHO try Google Checkout or PayPal. As per my experience Google has really cheap processing fees per transaction so it will become good choice for you. On the other hand PayPal is the most popular payment gateway.
You should also consider 2checkout payment gateway it is really nice option for you and you'll find most of the billing systems such as WHMCS and Clientexec. Etc.... provides it API as well. Even they also have a debit card option to access your finds more quickly.
Paypal is the most widely used payment gateway. However, if you want to use a payment gateway which verifies the payment very thoroughly, Google checkout is for you.
We have been using Paypal as one of the payment gateways and never found any issue with it. Its been a great payment processor. However, you can prefer Google Checkout as an alternative for processing payments.