Dries Buytaert compared different web server and PHP set up for a Drupal powered site. It turns out Apache2 + mod_php4 + APC cache is actually the fastest configuration. However, pre-fork with APC cache on each apache process can use quite a lot of memory as well, if the site has been hit hard. Personally for Drupal I still prefer lighttpd + spawn-fcgi which has much smaller memory footprint, and almost the same performance.
Scott Yang's Playground
Faith, Technology and Randomness in Life, According to Scott
Jun 9 2006