A good long term strategy to support the increasing workload [closed]

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http://serverfault.com – Let's make this simple. I have a website hosted on a single dedicated server (LAMP). All static content are alraedy on an external CDN. PHP pages aren't a problem because they take pretty low time to load (0.05). What is a good strategy if i see my dedicated server can't handle more users and I need to expand it? If i don't want to use a Cloud.. I can buy another dedicated server but at that point how can I split the traffic between my 2 dedicated server? Which is a good strategy for this? Note: yes i use a database of course (mysql) and no i don't store any session information (HowTos)