Sure, it may sit in the depths of HR until the end of time, but you can be sure of two things: One, your former boss (and future reference) will read it and be impressed. And two, if you ever decide to come back to your company (hey, it happens), it's a very good thing that last thing on file is a great, professionally written resignation letter.
A Professional Greeting. Your letter should be addressed to your boss. There's a good chance that this letter will merely end up in a file in HR, but it starts with your boss. Assuming this is someone with whom you work reasonably close with on a daily basis, it doesn't have to be super stiff and formal. Specific and professional is the right tone here.