In recent years computing power has increased to the level that 40 bit security can be cracked by brute force computing in a matter of hours. 40 bit encryption is therefore considered insecure.
If you are using the latest web server software and your web site visitors are using the latest browsers, a SSL Certificate is capable of delivering 256 bit encryption.
This level of encryption delivers the highest possible security, and using the same computation power that would crack 40 bit encryption in hours, will take billions of years to crack 256 bit encryption.