the lone sysadmin: When to Cluster, When to Build Big:

When you're building a medium-large sized service there's a Fundamental choice you have to make: do you build it on one big machine or do you cluster smaller machines? If you choose a cluster do you make it fewer, bigger machines or a bunch of smaller machines?

A nice breakdown of the issues around clustering - when to do it, when not to do it, how to go about it if you decide you need to do it, and factors to consider while making the decision.