|Don't forget too, for high use servers people use load sharing/clustering technology which allows multiple servers to handle the same data and share the load of users accessing.|
As wanderer said, there are too many variables to give a concrete answer. My suggestion is, if you find your server is getting close to running at capacity, look into a load sharing/clustering setup.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.