Is adding a second global catalog server a good idea?

We have a small SBS2003 network with a second DC, the first DC runs exchange, dns, etc. I would like to enable the users to logon to the network if the first DC is down or out or both.
Is this configured through global catalog? When my primary server is offline, users cannot logon to the network. What is the best way to configure for fault tolerance, second logon ability?