Do you have any warning message displayed in admin section? DB Cache Reloaded should display it if it will detect that wpdb class is already loaded.
Which WP version do you use?
Please also check your WP files. On my install (WP 2.8.4) following files are loaded before wp-content/db.php:
index.php
wp-blog-header.php
wp-load.php
wp-config.php
wp-settings.php
wp-includes/compat.php
wp-includes/functions.php
wp-includes/classes.php
or (for admin section):
wp-admin/options-general.php
wp-admin/admin.php
wp-load.php
wp-config.php
wp-settings.php
wp-includes/compat.php
wp-includes/functions.php
wp-includes/classes.php
Please check these files to make sure they are the same as in official WP archive (except for wp-config). If you have changed them (e.g. wp-config), please check your changes.