Making a backup server

Hi all

I hope you can help, i am working for a company and we want to make a backup server for our main 2003 sbs server, can i do a straight ghost and clone from the existing one or not, do i need to purchase an additional license because we will only use the clone server when the main server is down so only one will be running at one particular time.

Anyone had experience of doing this that can help me.

many thanks

mark