Keith Winston at Linux.com - How to configure a low-cost load-balanced LAMP cluster:

"The ubiquitous Linux, Apache, MySQL, and PHP/Perl/Python (LAMP) combination powers many interactive Web sites and projects. It's not at all unusual for demand to exceed the capacity of a single LAMP-powered server over time. You can take load off by moving your database to a second server, but when demand exceeds a two-server solution, it's time to think cluster."