I don't have an answer for you, but I have some variables you might consider:<p>1) how fast are you growing?<p>2) how fat is your profit margin?<p>3) what is the ratio of usage to capacity? peak usage? sustained usage?<p>4) how quickly and automatedly can you add servers?<p>5) looking at (3) and (4), what happens WHEN a server dies? can the rest of the servers cope with the load? do you have automatic failover? do you have any single point(s) of failure?<p>6) how much developer time are you willing to put into things like adding aggressive caching and fixing performance bottlenecks?<p>For the technical side of things, make sure you've read "Building Scalable Web Sites", by Cal Henderson. It will help you spend your resources much more effectively.