Practical Scalability

Hi folks We are currently designing a retail site that may need to scale up to a small server farm over the next year or two. Vendors of (expensive) commercial middleware have been trying to scare us with the difficulties of scaling with PHP. I turned to the leading PHP books for reassurance, but unless I am missing something the issue of multi-machine applications isn't really addressed. Discussion in the archives seems to have focused on much larger and more complex sites than ours, and much of the advice was along the lines of: "I haven't actually tried this but it ought to work in theory..." So I would appreciate advice from people with practical experience of the following: 1) An proven and robust strategy for scaling across three or four servers 2) The key factors we should consider in our original design to ensure that we can scale up from a single server quickly and easily once the need arises. We will be a fairly typical, read intensive/write light retail site. We will be using the the usual Linux/Apache combination with (probably) Postgres at the back-end. And although we are serious about learning good PHP skills, we are far from being trained software engineers. So we need a solution that doesn't require super-nerd type coding ability. We would be very grateful Geoff Caplan geoff@britishgoldsmiths.com