Microsoft has published a features comparison chart that shows the differences between the various editions of SQL Server. It should help you make the correct decision for your company's needs.
By the way, it's important to note that upgrading to SQL Server 2005 from SQL Server 2000 may not have any positive impact upon your application's performance. Before upgrading, make sure to carefully analyze the system in order to determine what is causing the poor performance. It is quite possible that you'll be able to fix the issue without upgrading.
Dig Deeper on Microsoft SQL Server 2005
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