Should the register form be an AJAX call or a traditional POST

I've been batting this one around in my head for a while and I decided I wanted to get other opinions as well. I am constantly writing registration/login systems and I can't figure out whether to have the registration page (upon successful completion of the form) to direct to the login page, or just keep the user on the register page and allow them to navigate to the register page.

Their account has to be validated before they can login anyways, which is why I would shy away from moving them directly to the login page, but then the question is, how can I show them a successful registration. Because in hallway usability testing, messages at the top of the page in green don't do it. But would a popup?

Excuse the rambli-ness, but my question boils down to two things:
1. Should a registration form be submitted via a traditional POST or via AJAX?
2. If via AJAX, what is the best way to alert the user that they have registered successfully?