Before you make additional hardware investments, you need to analyze your system for possible database changes that may be needed to speed up performance.
Open Manage | External Datasources and examine the external datasource references. It's very likely that you have some that need to be cleaned up as Version 5 tended to create very cluttered external data source references (and then didn't let you edit them!).
Check for scripts that run when the file is opened as that could explain your spinning beachball that holds everybody up each time someone opens the initial file.
For both of these tasks, a copy of FileMaker Advanced with its Database Design Report and Script Debugger will be invaluable.