Unless you know what you need to scale to, you can't even begin to talk about scalability. How many users do you want your system to handle? A thousand? Hundred thousand? Ten million? Here's a hint: the system you design to handle a quarter million users is going to be different from the system you design to handle ten million users.

http://teddziuba.com/2008/04/im-going-to-scale-my-foot-up-y....