Windows Server 2003 Disk Space

I have a Dell PowerEdge 2900 Server with Microsoft Windows Server 2003 R2 Standard Edition SP2. Storage is RAID 5 consisting of 5 x 146GB SAS (10,000rpm) 3.5 inch Hard Drives.

I am slowly running low on disk space and have 3 slots spare at the front. How easy is it to add 3 new drives to the system without losing any of the data currently on the server? I assume I would need to purchase 3 x 146GB drives?