320GB drives dedicated to the OS seems like a bit much, no? The power supply is also overkill, you don't need anything close to 850W to run that box. This could certainly be a lower cost build.
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.)