
My initial thoughts on the subject are:
* CPU speed not critically important + server will be idle in non-peak hours - find a CPU / MB combo which can drop to very low power states by linux when idling. MB should have no fans - bonus would be CPU fan which does not rotate when there is no need (ie. as in my GA 965-DQ6 + Core 2 Duo 4300, original boxed fan is not rotating at all)
* 1-2 TB HDDs - 2x - 4x Samsung HD501LJ SATA in RAID (hopefully they can endure getting into sleep mode and back when needed)
* lowest power and no fan GFX card (as a game developer I've got tons of old ones from beta testing)
* most efficient PSU for the measured range of power usage with room for 4 HDDs. PSU will be bought last after measuring components.
I'd appreciate any advices and pointers to useful informations about the subject.

ps. server will be 1-4hours "hot"-backed up using rsync by another one, with easy fall-back and fall forward, so there is no need for expensive and/or redundant components.