I am going to buy a NAS enclosure. It'll be used for backups from several machines (including Sooloos), and some streaming.
My question is: at what point does processor speed become relevant?
CPU/RAM will also affect performance of any background apps you have. I set up a QNAP TS10something for my parents for a Squeezebox and the performance of the Squeezebox server software was very poor. When it died suddenly I set them up a hands-off server based around Atom.
I used to use an old laptop under the stairs as a NAS. But now I've made it an additional task for the HTPC. The HTPC only uses 40W running so I just leave it on 24/7. It also runs an online backup client so is quietly uploading encrypted data at it's own pace. I also have another live copy of everything internally and also a delayed backup internally which is sometimes swapped for a delayed backup at another location. I reckon that should cover it.
Personally I wouldn't recommend any stripe based RAID (0, 5) for home use. It's too much risk and complexity. I know people who have lost data through RAID and I know people who have had NAS devices fail on them. If you have your data/backups in a standard format (FAT32/NTFS/ext2/[censored]/whatever) on a disc somewhere it's not hard to find a device to read it.