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How do you do Load Testing and Capacity Planning for Web Sites

I’m working on a community platform writen in PHP, MySQL.

I have some questions about the server usage maybe someone can help me out.

The community is based on JQuery with many ajax requests to update content. It makes 5 - 10 AJAX(Json, GET, POST) requests every 5 seconds, the requests fetch user data like user notifications and messages by doing mySQL queries. I wonder how a server will handle this when there are for more than 5000 users online. Then it will be 50.000 requests every 5 seconds, what kind of server you need to handle this?

Or maybe even more, when there are 15.000 users online, 150.000 requests every 5 seconds. My webserver have the following specs.

Xeon Quad 2048MB 5000GB traffic

Will it be good enough, and for how many users?

Anyone can help me out or know where to find such information, like make a calculation?