For a start you'll need to use SSL when taking the customer's card details.
You'll also need to be PCI compliant, e.g.:
Build and Maintain a Secure Network
Protect Cardholder Data
Maintain a Vulnerability Management Program
- Use and regularly update anti-virus
software
- Develop and maintain secure systems
and applications
Implement Strong Access Control Measures
Restrict access to cardholder data by
business need-to-know
Assign a unique ID to each person
with computer access
Restrict physical access to
cardholder data
Regularly Monitor and Test Networks
Maintain an Information Security Policy
- Maintain a policy that addresses
information security
Ultimately, you will want to avoid storing any card details without a serious compliance headache.
Using MasterCard MIGS should mean you don't need to store any card details - just send them under SSL.