I just posted a large excerpt from Ansible for DevOps over on the Server Check.in blog: Highly-Available Infrastructure Provisioning and Configuration with Ansible. In it, I describe a simple set of playbooks that configures a highly-available infrastructure primarily for PHP-based websites and web applications, using Varnish, Apache, Memcached, and MySQL, each configured in a way optimal for high-traffic and highly-available sites.
Here's a diagram of the ultimate infrastructure being built:
The configuration is similar to what many larger Drupal sites would use, and with the exception of the varnish
default.vcl and the actual PHP script being deployed (in the example, it's just a PHP file that tests the rest of the infrastructure and outputs success/fail statuses), you could drop a Drupal site on the Apache servers and immediately start scaling up your traffic!