If you have a separate physical backup of your data, then you can benefit from RAID 1. However, if the only copy of your data are the two HDDs in the DNS-325, then your data is potentially at risk if you use RAID 1. Using RAID 1, if you delete a file on HDD1, the data is instantly deleted on HDD2 - so there are no recovery options for accidentally deleted data. Additionally, if a virus attacks HDD1, the changes/corruptions are immediately copied to HDD2.
The DNS-325 does a have a Recycle Bin option that should protect from accidental deletions, if you can get that feature working properly.
I'm not trying to dissuade you from using RAID 1, just be mindful that RAID is not a replacement for a true physical backup.