adonet,
You need to import only the namespaces that you will need in your application. Importing more than needed simply hurts application performace. The one namespace you will need for all your apps is System. Asides from that, the namespaces you will import will depend on the kind of application you'll be building. For example, for database applications, you will need the types in the System.Data namespace. If you'll be building a Windows Forms (GUI) application, you'll need the System.Windows.Forms namespace, and so on. Again, you should only import the namespaces that contain types you will need in your application.
If you don't know the namespace of a type you want to use, there is a very nice tool that comes with the .NET SDK that allows you to do a search on such type. The tool is called WinCV.EXE (short for Windows Class Viewer) and its found in the "C:\Program Files\Microsoft Visual Studio .NET\FrameworkSDK\Bin" folder.
Hope this helps!
JC
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