[eluser]kanjimaster[/eluser]
Here are a few recommendations based on what I'd do ...
I'd use jQuery and the [email=http://jquery.malsup.com/form/]jQuery form plugin[/email] to greatly simplify the javascript.
Then you can build an ajax handler in Codeigniter in pretty much the same way as you'd load a page, except that the output will be returned to browser as an ajax response rather than a fresh page load (jQuery's ajax wrapper will take care of that for you).
It is a big, big learning curve. But once past it you'll be surprised at how little code you need to write to achieve all this - jQuery and Codeigniter do all the hard work for you, and nowadays a bit of an understanding of javascript is indispensible for anybody doing web development.