WordPress version 2 Solution
In older versions of WordPress prior to version 3.x admins could access the file edit-comments.php via FTP in the /wp-admin/ path.
hanging the 20 to the preferred results per page would then allow you to moderate many pending comments much faster.
WordPress version 3+ Solution
WordPress 3+ makes this much easier. From the dashboard > comments page admins can now simply click the ‘screen options’ tab near the top right, and select the number of results per page from the drop-down. Problem solved.