I myself use a Quadra 630 as a redundant backup server, in tandem with my Performa 5200 - and it works great. I put my old 4GB IDE drive into the Quadra and everything works fine.
I have considered putting NetBSD onto the Quadra (it is completely compatible) but I also have two IIci's that are already running NetBSD, so the Quadra will probably remain as a MacOS based simple server.
System 8.1 runs the best (in my experiences) on this class machine - I have had the thing up and running for over three months now without a single crash, error or problem of any kind (I do daily backups of several hundred MB's at night). Ethernet cards for this machine cost about $29, and I believe can still be gotten at http://www.macworks.com
NetBSD is a very mature operating system on the 68k Macintosh platform, and there is every kind of software and servers available for it. The MacLinux68k project website (very extensive) use to be hosted entirely on a Quadra 630 running NetBSD, but I'm not sure if this is still the case. (http://www.mac.linux-m68k.org/)