Server Sizing 2: Client-Based Calculations
A second approach to sizing is to monitor the existing client computer resource usage and attempt to extrapolate the results and apply them to the server systems.
Use Windows Performance Monitor and Windows Task Manager to measure the resources that the applications consume on the client computers where they are currently running, assuming that they are running on existing machines. Once this data has been gathered for all the applications that will run together on a server farm, it can be used to determine the amount of memory, processor, disk IO, and network resources that will be required to deliver a satisfactory Terminal Services user experience.
Ideally, run these tests on actual Windows Server 2008 ...