I would like to get more information about securely accepting and
authorizing customers' personal and credit card information over the
internet.  I want to set up my website that when a customer registers
(the customer doesn't need to make a purchase), the customer needs to
enter in a valid credit card and other personal information.  I want
to ensure that the customer's information is protected, and that the
credit card number is valid.  I am thinking about signing up with
GeoTrust to get a secure certificate.  However, I am not sure if I
would need GeoTrust if I go through a gateway like Authorize.net using
AIM or SIM.  Do I still need a secure certificate for my website?

More information:
1) The registration form resides on my server and I do not want the
customer to be transferred away from my site in order to complete the
registration
2) I want to make sure that when a customer registers, the customer's
personal information is securely sent to my server (SSL)
3) I want to ensure that the customer's credit card (necessary upon
registration) is valid (thus the credit card needs to be verified)
4) If the customer makes a purchase, I would like to be able to charge
the customer's credit card securely also

Please let me know if you would need clarification.

I think you are doing enough already to ensure the privacy of your
visitors financial and personal information. However it is not
mandatory on the internet to have your credit and purchase services
endorsed by verisign.com which charges a small fee to be associated
with them.

Verisign is well known internationally and customers have learn to
recognise their logo and trademarks. Even amazon and yahoo use
verisign.

Hope this helps.