One of the most important architectural decisions that must be done early on in a scalable web site project is splitting the data flow into two streams: one that is user specific and one that is generic. If this is done properly, the system will be able to grow easily. On the other hand, if the data streams are not separated from the start, then the growth options will be severely limited. Trying to make such a web site scale will be just painting the corpse, and this change will cost a whole lot more when you need to introduce it later (and it is "when" in this case, not "if").