Which OS are you running? You need to use the OpenManage software to "reconfigure" your 2-disk RAID 1 to a 4-disk RAID 5 (438GB) or to a 4-disk RAID 6 (292GB).  Once you do that, you will need to extend your partition into the new Unpartitioned Space.  If you have 2008 or later, you can simply right-click your C: drive and Extend.  2003 will require special software.  Also remember that you can only extend into Unpartitioned Space next to the volume you are extending, so if you have a C: drive and a D: drive, you would only be able to extend D: (or you would need to first delete D:, extend C:, create smaller D:, then restore data from backup).  
we have a 146GB system volume. 2 x 146GB  I have added another two hard drives (2 x 146GB) How do I increase the space of the system volume without having to create another RAID 1?
PERC H7000 how to expand RAID 1 Virtual Disk.