@ faugusztin : you"re right, my HDD are power-consuming (between 6 and 8W each, depending on their activity). It means ~ 20W only for storage. I plan to migrate on 2 Tb low-power hard disk drives in the future (probably next year) : i was looking at WD Green series.
Currently, my power supply is not perfect, but not bad. My old configuration was consuming 60W at idle, so I already had a gain of consumption !
The mainboard : the choice of an FM1 socket was done because of its compatibility with my PERC RAID card (I will be probably add this to the config in the future, for ESXi support).
About the global consumption : I'm not a quest for each watt gained. If I can get a lower consumption with undervolting, I will try to. But I won't be looking for an expensive power supply if I can hope only 2 or 3 watts gained. Currently, 1 watt consumed means ~ 1 € each year.
I'll have to work on undervolting now. But since this is a Linux server, I don't know how to start...