Check out the "Diagrams" node of your database in Enterprise Manager. It works okay, but wants to make any changes to existing tables immediately when the diagram is saved. This may or may not be what you want to do.
For this kind of work I usually recommend a third-party product. Visio works well, you can reverse-engineer existing database objects pretty-easily. xCase is also a good tool. The benefit to using a third-party product is that your model is external to your database, and changes aren't applied until you want them to be made. Pretty important, IMHO.
Dan LeClair
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