Even beyond medium popularity (depending on the type of site of course), you can move from a VPS to a dedicated server and handle quite a bit of traffic before

Even beyond medium popularity (depending on the type of site of course), you can move from a VPS to a dedicated server and handle quite a bit of traffic before you would need to use multiple boxes. (At least with some basic sysadmin skills, like using nginx instead of apache, or at least switching apache to the worker or event mpm and configuring it appropriately.) The next step from there could be moving the database to a separate server. You can get quite a ways without even worrying about load balancing/proxying/mirroring/etc.