Comment Some research material (Score 5, Insightful) 274
I salute you for your ambition and determination. I hope you get to realize your vision.
Now, as I read your question, I remembered an interview I saw a few days ago with Ben Kamens, one of the engineers working at Khan Academy, talking about scalability and things like how they manage their operation and the spikes of growth they have experienced in the past. It's a little light in technical details, but you may find it interesting: Root Access: How to Scale your Startup to Millions of Users.
One thing I'd like to mention is that when you hear someone else talk about the things they've done and how they have done it, it's easy to see it as an advertisement for a particular technology platform (AppEngine and other Google machinery in the previous video, for example), but that's not the thing to focus on. Whatever choices other people have made, the good thing is that their advice can be useful no matter what choices you end up taking. I know this seems like such a trivial thing to say, but evidence suggests that a number of people miss this basic concept, and then discussions quickly degenerate into pointless noise about concrete technologies, instead of the ideas.
I'd also recommend that you pay a visit to Google Developers youtube channel and type something like "scale" or "scalability" in the little channel search box. You might learn a few things from some really smart people who have confronted very real situations regarding scalability.
Best of luck to you, my friend.