I had to give a speech at the Karlsruher Software Council about AJAX some days ago.
Now I've posted the Powerpoint slides titled "AJAX – Hype and Reality" on my site at PowerPoint Slides on AJAX
There are really a lot of low level articles on AJAX that explain how to use te xmlhttp object. My summary on that: Have a look at the productiviy of yourself or your programmers and find a useful level of abstraction above the native xmlhttp objects.
Maybe you find the slides useful.