It can theoretically be done without re-installing W2k3, but it depends if the controller lets you. And, this being Dell hardware and a Windows install, if the drivers let you. (The controller can do all the RAID itself, leaving the OS none the wiser, but IIRC the official drivers tell Windows all about and Windows wants to put its paws in the mix. Theoretically, this is so you can use a Window app to alter RAID parameters without rebooting.)

Being a Dell RAID controller, in its BIOS, you might be able to extend the existing RAID arrangement. If it lets you do so without destroying the existing RAID, it will be a scary process because you will probably need to remove the RAID config and set it up again similarly, but with mirroring to the new disks. However, it might not like the fact the new disks are larger.

Oh, and the controller really is a "add-on". Dell can sell you most of its RAID-capable servers without RAID capability. In some of them, it is a card in special slot.

Wade.