on the accessibility issue - do you make AJAX a requirement for those pages?
the method I've found works the best is to use unobtrusive techniques.
ie; you initially create the website without considering JavaScript. then, after the site is working, you layer JavaScript on top such that it adds features to the site.
an example might be
nfgws.ie > Home
1. with JavaScript turned off, you are shown a menu which includes a link to a sitemap.
2. with JavaScript turned on, the menu gains some sub-levels thanks to Ajax.
Kae