Start by backing up your wife's business files - she probably will be very unhappy if anything happens to them. Once you've backed the data up, remove the disk, install the new ones and format as RAID1 and then restore that data.
If you're feeling brave - omit the backup step, remove the disk with your wife's data, connect it to a computer - either running linux or Windows XP and the ext2ifs file system driver - and verify you can access the data, and then install the new disks in the DNS-323, format as RAID1 and then copy the data across.
You do not want to be swapping drives in and out after creating a RAID array - and - don't think that with RAID1 you don't need to backup, you still do.