1) You could send the sidebar, footer etc
as json, and the content as html. [edit: an by json I mean javascript ;-) or a combination of a script-tag with a fancy-nav.js, and then pull in any additional elements via json/javascript urls]
2) "lot of bandwidth" - I doubt it. It's only the embedded html you'd have to resend -- and that gets compressed with the rest of the main html document. Everything else (images, css, javascript) gets cached via normal request caching.
