Here is a recommendation for sizing the SAP backend (SAP ERP) to serve the requirements from SAP ECTR.
Per SAP ECTR user working in CAD, we recommend 60 SAPS (SAP Application Performance Standard) and 300 Mbyte memory.
Per SAP ECTR user viewing only, we recommend 27 SAPS and 230 Mbyte memory.
See also http://service.sap.com/sizing for more support in sizing a SAP system.