I agree with Matt: difficult to completely take Greenpeace's advice at face value based on past experience but they do have a point about extending the life of the electronic equipment we own.

Otherwise, we as consumers can also make a difference and look for alternatives to the major brands. If you want a low power PC, I suggest you have a look at http://www.tranquilpc.co.uk/. I have one of their T2e boxes that I use as DHCP-DNS-Subversion-File server and its average power consumption is between 25 and 30W (compare that to the 100 to 600W stated by James above) and it is really, really silent. Their green credentials also go beyond the PC itself: the packaging in which they send you the machine is all recyclable cardboard, no polystyrene or plastic. Oh and refreshingly they have great customer service!