So, as TomTom said, the answer depends greatly on your requirements, and they don't seem very well-defined. Do you know what your requirements look like in detail, or is this still in an exploratory phase? Do you have hardware already available for use, or are you planning purchasing?
If you're still trying to determine your requirements, and especially if you'd need to buy hardware, I'd strongly suggest experimenting on a "cloud" provider like Amazon EC2 before trying to build on your own hardware. This will allow you to try several different levels of service to get a rough idea of your needs. I've done this kind of experimental scaling several times myself, and while it doesn't always translate 1-to-1, it's often been helpful.