Usually web projects start small, everything is on the one server. But if site becomes popular one server won't be enough.  

So, since I don't have any experience in this, I'd like to get an idea of:  

1. what kind of load means that I need to move my MySQL DB to a separate server and how to measure this load?  

2. what kind of load means that I need to add another web server and how to measure it?  

3. approximately, how many users can one server serve? 

I'm talking about simple PHP app on a LAMP server like this:  

 \* CPU: Athlon 3800+  

 \* CPU Details: 2 x 2.0 GHz  

 \* RAM: 1 GB RAM  

 \* Hard Disks: 2 x 160 GB (RAID 1 Software)