SBS 2003 and Disk Space

I am running SBS 2003. There is very little disk space left on the Hard Drive. I also have a Raid Array with four Hard Drives attached. Unfortunately, this array was never configured during setup of server, so they are redundant.

My questions are:

1. Can I utilise these “spare” Drives as external storage or can I/should I, at this late stage setup my Raid Array?

2. If I add an external Hard Drive to the server to free up space, what would be the best “stuff” to move across?