We have built out infrastructure to support about 50,000 users. Premature? Sure, but fairly reasonable for the price. Most of the server capacity consists of our Cassandra cluster which we are using to personalize user content...
Also, I'd recommend spending some time looking at your architecture, as you should be able to handle 50,000 active users with a lot less metal. The extra infrastructure can often be more expensive than a well-designed architecture.