We have been using an Ajax-based front end to our product for about a year now and the biggest lesson I have learned is that
you should really drop the X from Ajax... don't use XML as your data transport, use something lighter like JSON. As you add more and more actions to your application you add more and more XML processing and one of the slowest operations is building/serializing XML DOM. It has become our biggest front end performance hit.
I'd recommend using JSON as your data transport and treat it as a DTO on both ends, not the actual view. The backend creates the DTO and sends it to the front end. The front end converts it to JS (trivial with JSON) which can be used to manipulate the html to create the desired effects.
Hope this helps.