The choice is actually yours, but I'd opt for AJAX. Well, as things are at the moment, I'd even skip the XML part for the moment. As you know, web standards are undergoing constant revision and soon you'll find certain commands getting obsolete or incompatible with some browsers and/or servers. I actually use the following combination for my work at present:

Client - Javascript, XHTML (at least close to its standard)
Server - PHP
Database - MySQL

If you have the time to constantly revise your code, you can easily bring in XML. But before you do, first verify if you really need it.

All the best
Khalid