I am a huge fan of the Netgear (formerly ReadyNAS) NAS devices. I've owned 2 of them now, and am now standardized on the little 4-bay NVX Pro. It supports AFP perfectly as well as Time Machine, SMB/CIFS for any windows needs, iSCSI, NFS, RSYNC, etc. etc. iSCSI is a great option not to be under-estimated, though NFS is the more traditional backup model for Network storage. I backup several Apple machines to the single NAS, which is now running 4x3TB drives in the XRAID2 format (essentially RAID5). I can pull a drive in live production and swap it out and the RAID re-syncs without interrupting file transfers, etc. It's also got some nice plugin's of some open source tools like Subsonic server, Wordpress, etc. etc. They make a myriad of devices with the same basic software, just different amount of drive bays, enterprise vs. SMB market, etc. etc. Again, I chose the NVX Pro, but there are other options up and down the line-up. And no, I have no officialiation with Netgear, in fact on a certain level, my company competes with them :-) Lastly, remember, there is value in off-site backups as well, in case of fire, theft, etc. If you're really paranoid, like me, you could additionally mirror your NAS to an off-site NAS or other cloud-based solution. Of course, there's a cost for everything. Good luck with it.
This was a setup I used. An Xserve with OS on an SSD, The first 2 1TB drives Raid 5, for OS replication,
then a 14 TB Promise Vtrak Raid via Fiber for the users and files. We then had a 24 TB super micro NAS
that we TIME MACHINED the data from the Promise to Via ISCI.
Apple never supported TM for Enterprise backup but after 3 drives failed in the promise we dis a restore from the NAS ( 9 TB of data ) took just under 4 hours to do the restore and it WORKED FLAWLESSLY !!
Fiber might still be a little costly but another option not mentioned was the Drobo. We tried a small 4 bay one with 4 2 TB drives via firewire 800 and it worked pretty good.
Good luck and I would like to see what you results are .