Dino Esposito has a GREAT AJAX architecture article
in the September 2007 issue of MSDN magazine. It's a multiple part article and Part 1 goes into the basics of ASP.NET AJAX.
Whats really noteworthy in this article is that Dino drills down and shows you whats going on behind the scenes with an partial page request using ASP.NET AJAX and shows you some optimization tweaks you can make yourself to help performance a bit.
At the last talk I gave on ASP.NET AJAX, I had put in place the groundwork for people to see that ASP.NET AJAX makes incorporating AJAX into your existing .NET 2.0 web apps, but that simplicity comes at a price. One audience member put that together and asked if a partial-page request goes through the full page lifecycle and unfortunately I had to let him know that it does. This series of articles should help show how you can optimize the performance.