I still do not subscribe to the notion that you "need" two servers. If you are upgrading your server, it would be wiser to have ALL your data on the NEWER MACHINE.
Bert is ABSOLUTELY CORRECT when he talks about the hard drive configuration. In my server configuration I have:
1) 2 mirrored (RAID 0) 160gb Hard Drives. These drives are only for my OS and Applications NO DATA.
2) 4 750GB Hard Drives (RAID 5). This is for DATA ONLY all of the DATA THAT NEEDS TO BE STORED IS STORED HERE. Now the way RAID 5 Works 2 of the Drives are used for actually storing the data. One is for Parity, and the fourth is the Hot Spare.
My RAID subsystem is on an entirely different RAID Controller from the MotherBoard.
Another reason I would stay away from Multiple Servers, is Multiple Licenses (Server OS, Backup, AV).
If your OLD SERVER as a RAID, you can probably use it as your backup machine INSTEAD of a NAS.