To begin, you need to understand how your enterprise will use SQL Server. For instance, using the system as a Web-based back end entails a different configuration than when using it in a two-tier environment.
If SQL Server will be used to house databases for multiple applications, you must consider all the functions it performs before you configure the system. Record everything you can think of that the system will be asked to do, and then ask the requirements team the same questions. If the information is not readily available, you must do some research on the Web to locate all the information you can about the products that will use the server to store data.
Contact the vendors of the products that will be used on the server and ask them to provide application-load specifications. Also ask for client references. Often vendors supply white papers and sizing tools that you can use as a guide.
If the software that will use SQL Server 2005 is going to be designed in-house, talk to the developers to get a feel for the application's profile. Get the functional and technical specifications and read them thoroughly. With any luck, the developers will have a testing environment that they are using from which you can obtain some hard metrics. The key is to gather as much information as possible regarding how the users will use the system.
Use this information to document the software that will access the server, including the user count, and categorize the users to estimate the load each application creates on the database, reporting, or analytical engines.