You could use Linked Server to create a connection from SQL Server 2000 to the SQL Server 2005. For description see:
http://msdn.microsoft.com/en-us/library/aa213778(v=sql.80).aspx[
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Now you can use the linked server inside your trigger to make modifications to the SQL Server 2005 database. Just an opionion. Personnally I would create stored procedures on both ends to handle the logic for informing (from SQL Server 2000) and making modifications (in SQL Server 2005). Then the trigger in SQL Server 2000 would only call the local procedure.