Another thing on power supplies.  The power supply will try to deliver the power your system needs.  So if you have a 600 watt supply but your system uses 290 watts, you really aren't wasting electricity.  A 400 watt supply would work (and cost, operationally, the same.)

It's like using 60 watt light bulbs versus 100 watt bulbs.  The supply will provide much greater power than either bulb will use, but the 100 watt will use more power.

Setting a CRT monitor to turn off after 30 minutes of inactivity will save you some "power money."  You don't need that monitor on while doing those long renders anyway.