*ta da* (as if by magic he re-appears)
I think you'll find the real issue here is that you only started talking about your new system less than a week ago and in that time a few of us haven't checked in yet you've had 3000000 posts and 3032342352 thoughts and figured we were no longer interested in your exploits when in fact we are!!
First point - your case looks pretty awesome, your amount of space is insane and I'm jealous, I've been getting all the shows I like recently and my space has been disappearing rapidly so seeing where your thinking is coming from although I dont really want to fork out that kind of cash for it.
Anyway - well done on going hardware solution - excellent choice and I fully agree with your current solution and choices - expensive but a rock solid very profession and reliable way to go thats going to make your life alot easier and worry free.
Your choice of 500Gb HD's is also a good choice - 1Tb drives right now are the very limit of whats available and so are NOT good bang for buck - 500Gb is the sweet spot right now so your doing great there.
As for the needing a backup - I agree with your point - totally unnecessary, everything you have on those servers is easily (although time consumingly) replaceable (movies, music, etc). All the photos, documents etc obviously should be backed up seperately but they'll also easily fit onto a single 500Gb drive.
There's some software called 'fileback PC' that I use sometimes personally - it'll backup your files, but better still, it'll keep up to X copies of the files when backing them up so you could use it to save say up to 5 different copies of a file, document, edited pic etc just to really give you a safe, thorough backup. With your music as well - you dont need to backup all the actual files to backup all the data - just backup your MC Database and you can then use that to auto tag re-rips of your files if the worst should happen.
Realistically anyway - if your house burns down and you loose everything you own - are you really gonna care at all about your music? Re-ripping some music is gonna be the last of your worries.
If you want to give file safety as well, you could consider and probably should consider making the shares read only - all your films dont need file tagging - just database tagging, your music doesn't need tagging - just do it in the DB and say once a month/week whatever map an identical share to the files except one that has read permissions as well and apply all DB changes to the files then re-map back the read only share.
The only files that need read/write are your documents really and what you can do with those (and your MC DB) now you have a rock solid system, is setup a scheduled job on the server that creates localised copies of all those files to another folder that isn't shared every night.
With the Xeons, and Server motherboards etc in general - they're classed as enterprise level so they have better error correction built in I believe is the main difference:
http://en.wikipedia.org/wiki/Xeon <-- there's more info about the Xeon for you though.
Not sure if you've bought it all yet but this is an awesome project for you to read up and learn all about it for with a great hands on to really give you a feel for it. If you get this solution worked out and properly learn about everything your doing, there are a LOT of well paying IT jobs out there specialising in Data storage and backup - think about someone like google, gmail, hotmail, yahoo, etc (just the obvious ones) - they all need guys who specialise in what your doing now!
Anyway yeah - I'm back on board and you have a full supporter of the way you're doing it now (ps we never laughed, just cringed!! lol)