Most systems (overclocked and stock alike) very rarely draw more than 500 watts under full load (of course there are always exceptions and it varies greatly depending on your hardware)
If you want to save power it's better off turning off the pc when it's not being used. If it's slow to startup and shutdown we can also help with that.
Also note that if a computer has, say, a 1000 watt powersupply, the computer won't be drawing 1000 watts from the wall. It will draw whatever it requires up to a maximum of 1000 watts or whatever wattage the powersupply is.
If you are looking at a new machine, it is easy to build one yourself, and there are energy-efficient processors available from AMD. The next thing to choose low power after a CPU is a graphics card, ideally you want one that doesn't require an extra power connector and draws power from the graphics card slot on the motherboard.