Now that I've found an answer I'd like to request that How can I determine the ecological impact of a computer? is reopened. I hope that my answer, in addition to the one already there, demonstrates the validity of the question.
My answer is basically that environmental impact can be measured by energy efficiency, as defined by Climate Saver Computing, Energy Star and 80 PLUS. (Of course, there may be other answers and other organisations.)
I included examples of environmental factors to clarify what kind of question I was asking. Perhaps I should remove the phrase "What other factors need to be considered?"