OldDogGuy, You will just need to create another Raid 0 with the 750GB drive, as you had with the 146GB drive. Then assign it in the OS. With the 146GB drive, when you get the new one inserted, just select Reconfigure from the drop down menu and then select the original 146GB drive. That will cause the new one to mirror to the original and then convert it to a Raid 1.  Let me know if this helps.
I just recovered a server that had gone dead taking with it one the two mirrored 750GB SATA drives it was fitted with.   I am now booting from a single SAS 146GB that the PERC5i controller has setup in Raid 0, which I plan to mirror in a few weeks time when a matching drive arrives.  I do not plan to run beyond Raid 1 (mirroring). I wish to increase the storage capacity by adding the good 750GB SATA drive into the system but irrespective of which bay I put it into, the 750 GB drive does not show up as additional storage. I've reloaded Server 2008 on both 146 and 750 drives and can boot from either one mounted in any storage bay, provided that  I import the foreign configuration at boot up. However I prefer to boot from the newer 146Gb drive and use the bigger one for storage. How can I add the 750Gb and / or other additional HDDs to this rig?  
adding non raid storage to a PE2900