System sizing is an exercise often done by budget. A "standard" server is selected; the licensing costs for the operating system, SQL Server edition, backup software, and other utility software is tallied; and then some additional storage and memory costs are added. Then various committees whittle down that number and request bids.
After the parts arrive, the systems team assembles and configures the hardware and installs the operating system, sometimes even creating and assigning the storageall without consulting the users or administrators of the system. When that is complete, the database administrators (DBAs) install the SQL Server software and the system is off and running.
Although this is a valid method, it has some inherent problems. The variables involved in the use of the system are not known, so they are not considered during the build and configuration of the hardware and software. After the system has run for a while, these design flaws begin to show; by that time, however, changes are difficult (if not impossible) to make. What happens next is an evaluation that should have been performed at the beginning, with the dawning realization that your current configuration just cannot get there from here.
A better way exists to determine the best configuration for your server, storage, and other components. You can create a set of checklists that detail the use of the server, the SQL Server 2005 editions, and the hardware components that work best for each. You can then document the environment and combine this information into three choices. The three choices represent standard, enhanced, and optimal solutions, each incrementing in price. The important part is to use the information you have gathered to explain what the three choices represent in terms of costs and benefits.