Thanks all.
The horizontal scalability sounds interesting. Do you know where can I find more info about makin my application horizontally scalable, or is it more of a configuration of Apache/MySQL type of issue?
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To make your application "horizontally scalable", don't do anything to keep it from being so. Don't use shared memory. Don't use the local filesystem. Put stuff in the database.
Voila! You can now run one database and many webservers. As long as the database doesn't fall over, you scale.