Unfortunately SQL 2000 does not have any built-in encryption routines.
Your best bet is probably to write a front end and use the windows cryptography APIs to encrypt the card numbers. Or write/locate an extended stored proc that uses the same APIs
It's not something I've done myself.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability