SharePoint Best Practices for capacity management
Some highlights that are also applicable to the Project Server world are:
Capacity planning should be revisited repeatedly in all phases of a project as assumptions become more concrete and more detailed hardware and configuration decisions have to be made. Plan accordingly.
Be familiar with the tested Office SharePoint Server scenarios
Use 64-bit server versions wherever possible to enable scale-up scenarios and improve performance . Future versions of Office SharePoint Server will only support 64-bit hardware and operating systems (<- this is also true for Project Server)
Plan the monitoring rules that you will use to trigger the appropriate scale-out and scale-up options.
Be sure to stress test customized platforms before they go into production.