And sure, this could be an email (all of the same principles would apply), but a letter makes things official in every sense. It also gives you a prop to take along to the face_to_face meeting with your boss, when you let him or her know that you're leaving.
Prepare for It. Like the situation above, you should go into the conversation knowing how you'll respond if your boss asks you to extend your notice. If you've landed another job offer and have already set a start date, this may not be possible__so practice sticking to your guns: "Because I have a new opportunity lined up, my last day here is firm.