I am not sure that is any hard and fast etiquette regarding emailing colleagues, supervisors, managers and higher ups regarding your imminent departure.

I do believe that the initial telling of your leaving for another position should be made face to face with your immediate supervisor and then followed with an e-mail outlining the discussion and your last date of employment to go to your supervisor, human resources and your supervisor's immediate supervisor. This should be done in case the supervisor decides that you said one thing and/or he said another. (Cover your behind)

The word will get around fairly quickly, so if you wanted to send an additional e-mail, it may be best to just wait until the day before you leave or the actual day.

include the fact that it was a pleasure working with everybody and that you have made many good friends. Also that you have been taught many new and valuable lessons which you will remember forever.

Don't promise "to keep in touch" because somehow it just never happens. However, you should include something along the lines of how you would like remain in contact and offer your e-mail address to everybody in case they would like to contact you for any reason.

If there are any co-workers who you have been especially close to or worked exceptionally well with, you may want to send them a personal e-mail.