A utility named contrib/pg_upgrade allows upgrading to 9.0 from either version 8.3 or 8.4 without needing to dump and reload the database, which is normally referred to as in-place upgrade. A 64-bit Windows version was released, which allows the server to use more memory than earlier versions. It's still not expected that extremely large settings for shared_buffers will be useful on that platform. But this does allow you to be more aggressive with settings like work_mem than you were able to before.
VACUUM FULL has been changed to use the same rebuild mechanism that CLUSTER uses in earlier versions. This requires more disk space, but is normally faster and it avoids the index bloat issues that VACCUM FULL used to introduce in many ...
