If you want to upgrade to larger disks you just need to get the larger disks, put them in the NAS and use it to format them as RAID 1 (you must do this because, like all small NAS boxes, the 323 uses a software RAID).
Follow my instructions to find the good disk (the only difference is you don't need to mark which slot because they are not going back into the box). The good disk becomes your backup but it is formatted with the Linux Ext2/3 file system that windows can't read and will mess up if it tries. There are a couple of programs and an IFS that might work (depends on which version of windows) or, the simple way, boot your computer using a live Linux DVD (we actually use a Linux computer that has removable disk carriers to do this work), plug in the USB adapter/caddy mounted good drive and transfer everything over the network.
NOTE doing it this way is not fast, there are several points that will slow thing down, but it does save your data and get you up and running again.