If you are building a fan site and you want to amaze your friends with a mod_perl guestbook, any old 486 machine could do it. But if you are in a serious business, it is very important to build a scalable server. If your service is successful and becomes popular, the traffic could double every few days, and you should be ready to add more resources to keep up with the demand. While we can define the web server scalability more precisely, the important thing is to make sure that you can add more power to your web server(s) without investing much additional money in software development (you will need a little software effort to connect your servers, if you add more of them). This means that you should choose hardware and OSes that can talk to other machines and become part of a cluster.
On the other hand, if you prepare for a lot of traffic and buy a monster to do the work for you, what happens if your service doesn't prove to be as successful as you thought it would be? Then you've spent too much money, and meanwhile faster processors and other hardware components have been released, so you lose.
Wisdom and prophecy, that's all it takes. :)