Question: does your home server Need to be powerful for what it does? Would a mini desktop PC or a single board computer be able to do the same job as your current setup? Of your main concern is truly power savings, consider those options. Most mini pcs maybe draw 60 watts, most laptops only draw around 15-20 watts, most SBCs only draw around 5 watts max.

The best way to increase energy savings on your currently owned server would be to upgrade your PSU and not motherboard. Get a gold rated PSU certified by energy star as it has at least 85% efficiency. Invest in a kill-a-watt measurement device that plugs into the outlet so you an see exactly how much wattage you are pulling and how much you manage to cut down.