Well we all need to save $
I am building 'GREEN' PCs from now on. Not because I want to save the earth from "Global Warming", I just want to save money. At times I can have 5 PCs and 3 laptops ON, so that can hit my electricty bill.
The best saving is in the PSU, look for the 80+ sign and there are Bronze, Silver and Gold versions to show how much more efficient they are. Also down size your PSU from 650W to 300-450W. 400W will run most peoples PC, even with a bit of OC. Two of my PCs now use the Antac 380W PSU.
Do you need to run a big graphics card that requires its own power! Get the smallest that meets your needs.
You can also set all the power saving options in the BIOS and OS to save you money. Have the monitor switch off if you dont use it for a short time instead of having the screen saver come on. Set the PC to go into stand-by if not in use. Have the HD stop again after a short time.