This is a difficult question to answer. When you are shopping for a dedicated server, however, it is bound to come up at least once. RAM (memory), hard disk space, and bandwidth all cost money. It does not matter if you are purchasing your own dedicated server or using a remotely-hosted solution. These things cost money.
For the average user, many of the dedicated servers web hosts offer are more than adequate, and there is a good chance you will not need most of your resources in the beginning. Even if you have an exciting start-up web business that expects to take off very quickly, you will probably only use a fraction of your allotted bandwidth in the beginning. Over time, however, you may soon outgrow your current server.
Therefore, the correct question you should not ask how much of these resources you need. The real question that should concern you is how prepared you are for scalability. Will your web host charge you an arm and a leg if you need to add a hard drive, a stick of RAM, or several GB of bandwidth? This is something you should investigate before you sign up. The last thing you want is to be forced to migrate your well-established server to a new host because it is too expensive to simply upgrade. Consider this before you make a decision that could affect you for many years to come.