I've run a live cd in RAM. I believe I was using Slax (slackware derivative) and at the bootup, ran the command
That may work on other distros, not sure though.
Also, the distro DreamLinux has a button in the gui called 'quiet and cool' that slows the system down, using less power. It is also a live cd, but this function will work in a HardDrive install.