There are so many good database planning software but it can be choose according to your data strength & the uses of that data.. These two are main factor and many other factors are there which can affect that...
What database? For mysql, there's MySQL workbench - http://wb.mysql.com/ MSSQL has Microsoft's management studio - http://msdn.microsoft.com/en-us/library/ms174173.aspx
If you're using mysql I strongly recommend mysql workbench. It's free and with a click of a button you can convert your designs into real mysql databases!
You must use either Universal Database, Oracle Database Enterprise Edition, or Microsoft SQL Server 2005 Enterprise Edition SP2 as your database software.By default you should reserve at least 500 MB for the MCS database if you are using DB2 and at least 1500 MB for Oracle. Several hundred new layouts and themes of average complexity would not typically increase the total size requirement beyond a few MB.
If you can clarify that exactly what is your requirement for that.. So we can give the best idea for your requirement...!!