The 321 works beautifully as a NAS, storing and recovering data using Finder with Samba or FTP with no modifications.
However, from the reading I've done and playing with the software, it gets pretty intricate setting up the DNS to do Bonjour and AppleTalk to make it do TimeMachine (whereas I use a simple USB external drive on my "hackintosh" without any trouble at all). I did go through the whole process, setting up avahi to provide Bonjour and netatalk to do AppleTalk, just to see if it worked - it did, but it took a while and it certainly helped to have a Linux background.
You can google for it, but you'll find a good description/blog if you google "noulakaz" - it is essentially the same on the 323 and 321, remembering there is no USB port on the 321.