Oh, the other concern is that RAID 5 doesn't work very well on WD Caviar Green drives.
Should I go for another HDD (bearing in mind that idle wattage is important as the server is on 24x7) or forgo the redundancy and backup to external HDD (for selective data) instead? If so, JBOD will be ugly though with many drive letters. Any idea how to get a new single drive without having 1 disk failure bringing down the entire drive aka spanning (RAID 0)?
If I've well understood your concern, RAID 1 + 0 (striped set of a mirrored pair) should work (with somewhat better performance). You will need - obviously - 1 more drive.
About the other questions:
- About the client-server scenario, I wouldn't re-use a notebook AC adapter for that job, as I don't think there are enough powerful ones. Anyway, an 80-100W power adapter could be adequate.
- Still about the client-server model, there are server-style Atom boards with 6 SATA ports, like the X7SPA series by Supermicro.
- For gaming, a Core i3 2125 might be considered.
- About smaller enclosure than the Q08, if you don't need an ODD, you may look at the CFI A7879
Have a good luck.