Migrating Drupal 6 to pressflow? After a lot of mysql tunings, I intend to use Varnish as the last resort to improve my D6 site's performance which has become increasingly sluggish. I realized that Varnish will not work with D6. And due to lack of som contrib modules, I still can not migrate to Drupal 7 which, as I understand, makes migration to Pressflow pretty redundant.

I am using nginx + php-fpm as web server and try to avoid Apache at any cost. This approach has of course its downside, for example I could not make use of Boost module. Now my question is that whether pressflow works with my web server configuration? and whether migration is a good idea performance-wise? And Pressflow suites me, where can I find docs to make a smooth migration on Ubuntu server. 

Thanks for your suggestions.
I have a client who takes deliveries in the form of the sites/ folder and a database backup, and installs them in Pressflow.  As long as the Pressflow version isn't older than the Drupal version, we have never had problems.

There is no real migration.  Just replace the files, run update.php, and move on.  Two minor gotchas, though.

Pressflow error reporting is different, and a lot (and I mean a lot) of E_NOTICE will end up in the dblog.  You may want to adust error_reporting to exclude these.

Pressflow is API compatible with Drupal, but I have run into situations with anonymous sessions.  Occasionally, I need to manually call drupal_start_session, which is in Pressflow 6, from a custom module before I start doing stuff with $_SESSION.

Personally, I would give Zend Server CE a shot, too.  The Pressflow 6 site that I am wrapping up really flies in this environment.