Let's do some math.
Say your machine draws 250 Watts (~2 Amps). That's 6 KilowattHours(kwh) per day.
The US. national average for electricity is $0.112/kwh (as high as $0.31 in Hawaii) http://www.eia.gov/cneaf/electricity/epm/table5_6_a.html
Electricity for your machine will cost 6 * $0.112 = $0.67/day (on average)
These days you can get integrated Atom boards with up to 8 SATA ports.
Less power, less heat, less space. http://www.mini-itx.com/store/?c=2
To give you some math back, my time on salary comes to a net of (approximately) €20/hr (and yes, it'd be far higher if I was contracting, thanks). Lets say a hard drive goes because I bought a different PSU which was closely matched to the spec but it's a friday model so the 12V line was kinda struggling. Now I have to buy the new hard drive, have it shipped, install it and rebuild it; that's going to come to 1-2 hours of my time in total on repairs, and probably three to four days where I don't have my infrastructure here which comes to around 24 hours of my time where I'm being put out because of a hardware failure. That's €480 worth of my time, against about $163/yr in running costs (because I really doubt this will be pulling 250W continously). Think of it as health insurance - yeah, odds are you'll never get back services equal to what you paid for, but that's not really the point...
Also a benefit of the low power draw, a 700VA UPS can keep that thing running for about two hours.
As to the 850W, darn right I don't need all of it right now, but (a) I want the PSU nowhere near its limits when running two RAID arrays; and (b) I'll be adding in disks in the future to increase storage space as needed; I don't want to have to buy another PSU or worry about stressing the PSU at that point. It's relatively cheap future-proofing.
You made the right call on the drive, low end hard disk prices hit a wall pretty hard which leaves them looking a bit silly.
You're vastly overestimating the amount of power required to run the disk arrays, even at peak spin-up loads. The only way you could fit enough hardware into that chassis to come near using that PSU is multiple high-end GPUs.
If you're going to build a home server it's almost always worth investing in an AC power meter if you don't already have one.
Nearly every PSU operates most efficiently somewhere in the middle of it's power range, operating a high power PSU at 20% capacity will use notably more power than a correctly sized one. Take a look at any PSU review over at silentpcreview.com, they do good efficiency testing.
One note from a physics major who's first job out of college was a the electrical officer on a frigate: this applies to all electrical devices. This sort of "over-supply solves all problems" attitude is quaint when it's a home server. It really sucks when under-loaded 1MW diesel generators shit the bed almost weekly on deployment in the summer. Any idea how much hot-dark-and-quiet sucks when you're bobbing like a cork in the middle of the ocean in the summer? Now imagine you're the person that 250 hot, cranky people expect to fix it. Why? because some "contracting expert" in the Pentagon decided to "over-spec" the generators 25 years ago.
Also, the lower you load it, the easier it gets to cool, according to its own specs. Which I take as a nice bonus.
The exception, I guess, might be for extremely fast platter drives or for SSD's. (And with platter drives, you're still to some extent up against speed by virtue of rotation speed versus speed by virtue of data density.)
Ex-enterprise gear or a retired/second-hand PC would be a lot easier on the wallet (and much higher quality for enterprise gear). Add a couple new drives in a mirror and you're good to go.
Low power 1U atom or amd fusion builds are the one place new hardware makes sense to me for a home server.
And yes, Atom builds could handle NAS loads easily enough, no disagreement there; but add in the backups and other tasks, and you'll start reaching the limits of the platform. This isn't just a NAS box, this is the main server for the house.
NAS, backups, web development, etc aren't going to stress an atom. I/O will be your enemy far sooner in most conceivable server scenarios until wide RAID arrays or SSDs enter the picture.
And buying an older system on ebay and shipping it... er, no, I wouldn't be doing that locally in Ireland. I'd get an overpriced, heavily kicked system unless I was very lucky, and to quote a google sysop, hope is not an acceptable plan :D
As for the other comments about environmental friendliness, well it's not that bad. Running old enterprise gear will more than likely use a lot of power. The processor and power supply in that build are a bit overkill, but I think it is overall ok. You could get a 45W version of the processor and I bet that power supply won't be using more than 10% of its capacity, so yes it might have efficiency issues. 80Plus certification only measures efficiency at certain loads, 20% being the lowest, so who knows how efficient that will be at 10%. But of course you could add a GPU in there, or run a few virtual machines. I still don't think Atom's are worth it for general computing, as the platform is pretty limited in terms of expansion (no PCI, seriously limited RAM).
Additionally, what you want in years to come might not even be the same thing that you want now. Two years ago, I sure couldn't predict what I'd like my machines to be able to handle today. "What? A web server? Whyever would I need that?"
When it comes to hardware, buy what you need right now and no more. Let future-you worry about himself.
Unrelated tangent: why is this text area so small, anyway?
I understand not wanting the hassle of buying a new drive at some point in the future, but I've a hunch that this is the sort of "future proofing" that makes your system seem charmingly quaint a year from now.
But you're wildly overengineering for your needs; you could literally spend half as much on a system which isn't "future proof", and in two years replace the entire thing for half again what you paid. So my question, which seems to be the common one, remains, why?
(b) This box's destiny is to go sit in a room quietly and just work. Any time I spend on it after it's in that quiet room is time wasted, and I'm willing to spend a bit more (though not a huge amount more) now to save a lot more then. If I was a postgrad again and my time was low-cost and I wasn't hosting data I really worried about, sure, I'd pick the newest, shiniest toy and try to make it work and if it didn't, oh well, I learnt something; but that's not the case here, I just want a solid box I never have to worry about no matter what else we throw at it. This ain't the project car in the garage being endlessly worked on for fun, this is the family estate wagon that's used for the shopping and getting the kids to school, if that analogy makes any sense.
Things it does: NAS for large file storage and backup, pulseaudio sink, transmission w/ webclient, IRC client (ssh/screen), light home webserving.
I used to have a more serious box, but I found this runs silently, very low power, does everything I need, and is tiny. Also I got it on woot for $20. I'd strongly advise a similar setup for home stuff.
I also love the fact that it also runs on less than 12.5 watts!
Just a light, wifi, touchscreen (with mic, speakers, and cameras) that uses my desktop, and just streams content to the device. A smart charging stand that hold it vertical, and you could use it in place of a traditional monitor too. (Pie in the sky territory: awesome remote abilities that let you control your desktop over wifi from anywhere.)
Oh come on downvoters. We're talking about $1300 for a glorified print and file server. Please tell me I'm not the only one that finds that surprising.