In your original question -

How to securely store credit cards to make them safe in case of a system penetration.

Don't forget the low tech, Kevin Mitnick style, social engineering. If you get enough credit cards, as previously noted, you become an attractive target.

Consider:

• Who has physical access to any system that has these numbers?
• Are the passwords written on the monitor?
• Where and how secure are your backups?
• Are there operators who have been warned against sneaky people calling up seeking "help"?
• Is the server room decently locked up?
• Does just anyone have the ablity to come and go?
• How good is the security of the back end that clears the transactions?
• Do you actively patrol the security updates for the OS?

Sometimes the fancy lock pick is a crow bar. -Diskcrash