I worked on some websites where they have two servers: one dedicated for serving php web pages, and another dedicated for serving media such as images, flvs, mp3s etc..When you do a view source of a page on http://www.mysite.com, you will see code like (img src="http://images.mysite.com/helloworld.jpg" ) where http://images.mysite.com is a different physical server from www.mysite.com.
Let's say I have to recreate an existing website that currently does not have separate media and web server. The existing site is a poor attempt at doing youtube. The existing site is reasonably popular with 2000 UV visitors per day and have monthly transfer of 250 gb. In re-creating the site, should I plan to have a separate media server? Should I plan to have 2 load-balanced front end servers? Or is this a "build as you need" scenario?