Big PSU's are a little big of a false economy (@ minyazz). The higher the wattage doesn't always mean the more it can power. (Please skip the next few setences if you are au fait with PSU's, I don't want to patronise
). e.g. a 520W Corsair HX PSU will probably supply energy to a high clocked, component filled system better than a Jeantech (random) 850W PSU. Good quality components dissipate less power themselves which reduce efficiency and the actual power supplied is cleaner.
Also, remember efficiency ratings. Most good quality PSU's operate about 80-85% efficient at their peak power usage. Now, consider this, at low power draw e.g. 100W a high wattage PSU will be much less efficient than a 300W PSU. Why? Because usually PSU's operate at their most efficient when at 50% load or so (can vary though range of 40-70% load say - it really dones depend on the actual PSU itself). An 850W PSU will operate at say 50% efficiency at 100W (an example), whereas even if a 300W PSU will operate about 70-80% efficiency at this load.
Doesn't seem significant yet?
My system is in my sig. It is custom watercooled and has a 18W pump and 3 120x38mm fans on the radiator. At load, (quad prime), it only draws 300W from the plug. At idle, it sits around 180W. My spec is quite a power, consuming one (I have 4 hard drives also, and many USB devices).
Anyway, to cut a long story short, ensure you pick a PSU for the purpose. In many cases, a good quality 500W beast will power even the most draining of systems, though 600W+ is better for those with mutliple graphics card, especially the uber ones out today! Even looking at your sig, I doubt you even hit more than 325W at load. Graphics may hit closer to 375W., if hitting the CPU at the same time).
Desktop hard drives only consume about 5-10W when idling/working. The real thing to consider is their power draw at spin up which can be 20-25W. This is why multiple hard drive systems sometimes can cause problems on startup for PSU's (not in any home environment really) and is also the reason why BIOSes offer options for staggered spin up.
OP: Sorry, I'm rambling! Just thought I'd add some useful information. You can build a decent server from even new components. I have a server running OpenSolaris and use this as a fileserver for all my computers in the house. It runs a RAID and is much better than any prebuilt NAS. It is an E2220, 1GB RAM, 20GB laptop HDU (OS), 2x 320GB HDU's RAID1 (storage) and draws 45W at idle and 60W working (normal fileserver use, i.e. writing to disks across the network). Not bad, considering it doesn't draw much more than an Atom setup, yet is FAR more powerful.
I definitely recommend FreeNAS for an easy setup but look at other things you might do with it. Sorry for the long post!
