Article : How do you support 500,000 users with Drupal?

Usually, the person asking is just getting started and the really struggling with the question of how much resources, financial and time wise, should they invest upfront, and how much should they invest to handle their ongoing growth.

There are a number of rules of thumb, but this question does a lot of facets. I'll do my best to pass on some of the same advice in this post that I give to clients and friends everyday.

I have 500,000 page views per month, but I'm expecting 10 million
The vast majority of customers make the mistake of building their infrastructure for what they think is possible, but not their immediate needs. This is a mistake most of the time but not always.

The rule of thumb for existing sites is that you want to build your infrastructure for six months to a year's worth of traffic growth based on existing trends, the trends are generally unreliable due to the geometric nature of traffic growth once your quote unquote plugged-in. Generally you wanted to a longer-term investment only when you have to or when the complexity of implementation is very high, such as in the case of implementing San or NAS like NetApp or similar.

The importance of horizontal scalability
I cannot emphasize this enough: the specific box you buy or the virtual server that you lease from a hosting company is not an important purchasing decision. It isn't. What's important is that you have a strategy to scale that virtual server or box as your traffic increases. Many of our largest customers started on a 256 meg xen virtual server, which is easily scaled by adding more memory as your traffic increases. So, you can start at a very small commitment, and not be spending a lot of money while your traffic doesn't justify it.

In fact we've had great success building infrastructure with virtual servers and load balancer, especially with xen and vmware.

It's entirely possible to build a robust fairly scalable hosting infrastructure that is entirely based on these commodity tools. 
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Courtesy : Workhabit.com