Keep in mind that 3200 people connecting to your application (web or thick client), don't mean 3200 connections to SQL Server. Usually those connections come and go.

However user connections don't give you any idea for load unless you know what they're doing. One user can fill tempdb or use up all the memory.

What you need to do is profile the application, and get an idea of the load from different use cases. Then scale those up. SQL Server 2005 can definitely handle large loads, though I'm not sure why you wouldn't at least use 2008 R2 if not 2012.