Even if WordPress 2.7 will be released soon, I still upgraded all the blogs and sites that I am maintaining from WordPress 2.6.3 to WordPress 2.6.5. There is no WordPress 2.6.4 to avoid confusion when someone spread a fake release of WordPress 2.6.4.
WordPress 2.6.5 is a security fix solving one problem and three bugs. You can download it from the site and perform the normal process of upgrading or opt to just upload the 5 files.
wp-includes/feed.php
wp-includes/version.php
wp-includes/post.php
wp-admin/users.php
xmlrpc.php
Kindly read the official announcement about WordPress 2.6.5 at the WordPress.org Development Blog.
For more information, WordPress 2.7 will go live at WordPress.com tomorrow. If you don’t feel like testing WordPress 2.7 Release Candidate 1 in your own site, you can take a look at it there if you have a WordPress.com account. It may not be entirely the same with WordPress 2.7 that will be available at WordPress.org but at least you will have an idea what is coming.