I completely agree to use Multiple Update Panel rather than to use single update panel,When you want only a certain part to be postbacked
if you want entire page to be postback then it is better to use single update panel.
Make sure that you make the updatemode="conditional" for all the updatepanel otherwise all the updatepanel will get refreshed.
Also Check out the below Post for complete usage for update panel
http://www.codeproject.com/KB/aspnet/Select_List_Box.aspx
