lower power usage?

I got a problem here.
In my house my family just got a ton of new computer systems. 6 desktops in total, with three of them being dual CPU systems that eat up a ton of electricity. One of those dual CPU systems is mine, and it probably uses up more power than any other system in the house.
The power bill just came in and it was around $400, so the computers can no longer be running all night downloading, but I also can't max out my torrents during the day cause people actully use the internet at my home.
Is there any way to lower power consumption and still download the torrents?

Is there any reason why you can't just leave *one* computer running during the night? One computer (mostly) idling for 8 hours extra per day should not add a significant amount to your electricity bill.