If at all possible, start with md RAID1 right away. If need be, set up a single hard-drive as a RAID1 member. You can then add a second drive and get the redundancy without even rebooting (provided your hardware supports hot-plugging). Even resizing a RAID/LVM partition is fairly straightforward. But turning a regular install into a RAID1 install sounds like trouble (I've never tried).
I didn't know it was possible to configure a 1 drive RAID 1... Well, maybe that's dumb, but I'd prefer to get the 2 drive on the same time, so that they'll be fresh. In the meanwhile I'll use a 500 GB drive.
As to hardware, a budget silent file/BT server is pretty much an Atom unless you want ECC RAM. I don't know what a "DNLA server for a PS3" requires but, if an Atom can't cut it, your old computer might not cut it either (or at least no without yet another upgrade to handle video compression).
Well, I don't really agree. Having a DLNA (or UPnP) server means to have to transcode sometimes the videos so that they might be readable by the client (here, the PS3), and it requires CPU horsepower, which the Atom platform lacks unfortunately.
Your idea of salvaging a used computer is laudable but consider the price of the parts you're considering and compare that to a the price of a D510MO (motherboard, CPU and fanless heatsink are all included in that price). Some Atom boards take power directly from a brick if you don't want to buy a fanless PSU or a case that comes with one.
If you need some kind of graphical horsepower for your application, you could buy an old-generation Atom/Ion or possibly a dedicated card that handles video compression in hardware.
If I go for a D510MO (which I considered a few months ago) I'll still have to go for the PSU so I'll have to pay more for something that is not as powerful. And will the MoBos with integrated PSU be able to handle the RAID and possible evolutions? I'm afraid not, it' usually sized for a 1 Atom + 1 hard drive + 1 optical drive usage.
Consider also how cheap nicer low-power gear is. The Proliant Microserver has a somewhat noisy fan (at stock speeds anyway) but it shouldn't be too hard for you to change it if you're into DIY. You get very low power consumption (lower than a current-gen Atom with integrated graphics according to my UPS), ECC RAM support (claimed, not tested), a slow CPU that still beats Atoms, a halfway decent low-power graphical subsystem, a decent NIC and, last but not least, 4 hotplug bays for 3.5'' hard-drives, all in a well-designed small case. Plus an HP warranty. It would of course cost you more than silencing your old computer but how much better would that be as a server!
That's indeed an interesting one.
I'm all for re-using old gear but only with cheap parts. Or better yet: really cheap, second-hand parts!
I agree. But (except for second hand), the cheap parts are usually not quite