It depends on how you plan to set up your NAS. If this is a backup device, do you want a RAID 1? Do you want fault tolerance from say a power failure which could cause data corruption? Do you demand speed?
Two key things you must read about...
1. EXT2 or EXT3 format for the drives.
2. RAID 0, RAID 1, Seperate volumes, or JBOD.
Do a google search for those items and make up your mind. One thing to note, this little NAS is not a speed demon. It's good for a home NAS.
-Joe