You several options to consider.
You can format the DNS-325 as two standard Volumes, where HDD1 contains your source data and HDD2 contains the backup. Using this approach, through the DNS-325 you can schedule nightly (or weekly) automated backups between the two volumes.
This approach will give you more control in the case of accidental deletions. Remember, if you accidentally delete a major parent directory, there's no "undo" button. With RAID 1, the data is instantly deleted from both drives. So, unless you have another full backup of your data, using RAID 1 as your only data storage could result in disaster (I strongly discourage you from doing this, but the decision is yours to make).
Personally, I have two DNS-343s where one unit holds my primary data, and the second unit backs-up the first.
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Regarding keeping or removing your data on each PC. . .
I keep my data centrally located on the DNS, where each PC on my network directly reads/edits the data on the NAS. Adopting this approach will force you to stay more organized and is easier (IMHO) to administer. This is my personal preference and is something you need to evaluate on your own. Again, unless you have a full backup of your data, this is a risk-ladden approach.
A second approach is local storage on each PC, where you schedule nightly or weekly or ad hoc backups from each PC to the DNS. This approach works well for laptops that are frequently off the network, but need to be synced up periodically.
If you have a large media library, your best bet is to maintain the master copy of your data on the DNS-325 (as this is a task the unit is designed for). Managing your photos and media files on the NAS makes organization much easier and makes sharing data among other devices on your network easier as well. Again, make sure you have a full backup of your data, and remember, RAID is not a backup, just a means of providing redundancy.