Windows 2003 & SQL licensing

We need to add a SQL server to our existing Windows 2003 network. Currently the network consists of a windows 2003 server -  30 user cals. We intend adding a new server which will need to have an SQL processor license due to the software application requirements. The Operating system for this server will obviously be windows 2003 BUT how may user cals must it have???  Are the Windows 2003 standard 5 user cals sufficient?

Thanks in advance