In most cases, you need to have the manual submit, even when you've utilized AJAX to submit your forms. JavaScript is not always enabled for a variety of reasons. Most text-only browsers usually don't interpret JavaScript and so it's important to have a fallback. Also, although I know it's not applicable for most forms, search engines have very limited ability to parse and execute JavaScript, so it's best not to use it to construct menus.
Is it better to enhance a form with AJAX? It depends on your situation and also on your implementation. If all you're doing is an AJAX form submission, I'd recommend against the extra 20K overhead of a JavaScript framework like jQuery or MooTools. In that case, use pure JavaScript. However, if you're using the framework elsewhere as well, it could be a good choice. Just be aware that using AJAX can present a number of usability issues (like the back button behavior).
Also, just as a warning and friendly reminder, don't just do your validation with JavaScript. It needs to be done on the server, first and foremost. Use the JavaScript to enhance the security, not to provide it.