"If you've setup your drupal deployment with a separate database and web (drupal) server (see scaling drupal step one - a dedicated data server), a good next step, is to cluster your web servers. drupal generates a considerable load on the web server and can quickly become resource constrained there. having multiple web servers also increases the the redundancy of your deployment. as usual, my examples are for apache2, mysql5 and drupal5 on debian etch."
http://www.johnandcailin.com/blog/john/scaling-drupal-step-two-sticky-load-balancing-apache-mod_proxy