Everyone should keep this in mind when they worry about server architecture - SO runs off 12 servers. As long as an app doesn't do anything crazy, it's going to scale very easily throughout the first few years. 1 server should be enough for almost anyone. 1 database, and a few frontend web app servers will take you to millions of views per day.
Unlike user software, most web apps get faster every year, and the stacks tend to get faster (due to better interpreters) not slower (due to silly 3D effects on the desktop).
pg loves to say that people who pay for server software are crazy. That might still be true for Google these days, but not for most people. You don't need many servers, and Windows is no longer slow.