Technically, your farm exists as soon as you create the configuration database with the first Complete installation type. Depending on your design, you can immediately begin to expand your farm with additional servers to support WFE or other functions. You might need to move search, index, personal sites host, Excel services, and other services to separate hardware to reduce workload on the WFE.
Alternatively, your farm can continue for some time as a single server hosting all services (other than SQL Server) until performance monitoring indicates a need to expand. We recommend that the SQL server always be on a separate server except in very small production implementations or development and staging implementations.
The installation process is the same for additional servers as for the first server in the farm except for the installation type selection that you make. Your only options are Complete or Web Front End. You can choose to always use Complete and turn off services not required for the WFE server when installing a WFE so that you retain the flexibility of changing roles later. If you use the WFE option and choose to add other services to the server later, you must reinstall SharePoint Server 2007 to add the new functionality to the server.
When you run the Configuration Wizard, however, you must choose to join the farm by picking the appropriate configuration database that defines the farm. The information in the configuration database will be used to configure SharePoint Server 2007 on the new server. The existing configuration database names can be retrieved after you identify the database server.
After the Configuration Wizard completes, you will define the role of the new server in Central Administration by starting and stopping services. If you're coming from a SharePoint Portal Server 2003 background, you'll find that there is no server selection screen with clean check boxes. Instead, starting and stopping services in Central Administration on each server is the method you'll use to assign server roles in the farm. This is much more granular in defining roles, and the hard-coded farm structures of SharePoint Portal Server 2003 are gone.