Expanding Raid 1 Volumes when adding bigger drives

I have a Dell PE R300 Windows 2003 with a SAS 6/iR controller and  300GB mirror drives.  The current configuration has 2 volumes OS and a Data.  I have replaced the 300 GB drives with 450 GB and I am trying to figure how I can utilize the addition space for the data volume or create an addition volume.

Server Administrator (Ver 5.4) is showing the Capacity as 418 GB, Used RAID Disk space as 278 GB and Available RAID Disk Space as 0.00 GB.

Any suggestion would be appreciated..