I don't have an answer for you, but I have some variables you might consider:
1) how fast are you growing?
2) how fat is your profit margin?
3) what is the ratio of usage to capacity? peak usage? sustained usage?
4) how quickly and automatedly can you add servers?
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?
6) how much developer time are you willing to put into things like adding aggressive caching and fixing performance bottlenecks?
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.
