It sounds like you need to generate a Historical CPU and Memory Report using Report Writer. When you open Report Writer, as directed in "Creating Reports" (spec. the sections "Creating and Modifying Reports" and following) in the Orion NPM Administrator Guide, you will find a predefined CPU Load - Last Month report under the Historical CPU and Memory Reports heading in the left pane. The directions in the Creating and Modifying Reports sections should direct you through appropriate customizations. Save the report to an appropriate location.
You will also want to set up a weekly scheduled report using the Orion Report Scheduler, as shown in the "Using Orion Report Scheduler" section in the Orion NPM Administrator Guide.
Somehow I missed that historical report. Now I feel dumb ;) Thanks for the quick response.