A high quality safe from a brand like Ft. Knox is a must for any significant collection. I'm not a fan of visiting my cards at a bank or somewhere else.
http://www.ftknox.com/redesign/safes/guardian.htm
For any decent sized safe I recommend a de-humidifier rod that plugs in and sits on the bottom of the safe. You can buy them online from many retailers. It heats up and causes the air to circulate inside.
Moisture damage can't be reversed, so I don't take chances. These rods are cheap and you can just forget about the issue.