Before you choose and install hardware, it is best to run a simulation. Your hardware vendor or RDBMS vendor can help you to simulate a large number of users on a given platform with a given RDBMS.
Setting up and running a simulation that includes your own application lets you anticipate when moving into any installation, and can help to mitigate costly consequences.
A potentially costly mistake is to test in your office with a small database and a small number of users. Databases respond differently to small amounts of data and a small number of users than they do to large amounts of data and a large number of users. Functionally, the database works differently as the load increases. You might think of this adjustment as switching into a different gear. Significant changes in the load on your database can lead to large increases in overhead. The behaviors and loads encountered as the database expands cannot be determined from a linear projection based on the smaller scenario.