Okay, one more time...
I really suggest a search on this because it's been discussed on this BBS many times, but since you're here now:
The federal copyright laws are a codification of artists' existing common law rights, which have existed for centuries. They provide an orderly process for registering works and provide a forum in federal court for seeking relief against copyright infringement.
Each of us has a copyright on our songs that exists from the moment we create them. That copyright is infringed when somebody uses our songs without permission. In order to prove copyright infringement you have to prove, generally, that they had access to your song and that their version is similar to yours.
However, unless you register your copyright, you can't sue in federal court, which has big procedural and jurisdictional advantages. (It doesn't matter at all how long it takes to get a certificate; it's registered and protected the day the copyright office receives it.)
If you can't sue in federal court, you have to sue in state court. The idea of the "poor man's" copyright is that the postmark will prove that you have created your work prior to the time that someone else came out with it. It's evidence. That's all. It might be helpful, but it won't get you into federal court. By the way, if you can prove that your song was posted at MP3.com -- which you certainly can prove -- that would be evidence that's a hell of a lot more powerful than the postmark thing.
Don't tell anybody, but I'm a lawyer. Don't tell anybody this either, but I haven't copyrighted my songs yet. But I could represent myself. If somebody tries to publish my songs without permission, I'll make a lot more money from the lawsuit than from some measely royalty checks such that an unknown songwriter would receive! (so go ahead, make my day) But since you can't represent yourself effectively, copyright your songs -- anyone who can afford guitar strings can afford to do this -- and in the meantime, don't worry.
I think that about covers it. That'll be $500.