Dude, I love this question and hope you get lots of great answers (I'd love to learn from it too, this is just about where I'm finding myself at this point) but dragonchild is speaking gospel. If you have to ask these questions, you're probably not ready to do the project (at least not alone). I'd say, go slowly and try to emulate other successful, *similar* architectures.

As far as books go, I just picked up Building Scalable Web Sites and Scalable Internet Architectures a couple of weeks ago. I find the first to be the worst O'Reilly title I own -- I'm only halfway through and having trouble forcing myself to read the rest -- packed with gems like: "here's a great technique, but it doesn't scale" and "you don't have to consider race conditions in your architecture as long as they're pretty rare." Scalable Internet Architectures I like much better. A good road map for real definitions and real planning. Flip through it at the bookstore and pick up a copy if you like it.