Ben Armstrong reports a wonderful online tool for capacity planning on HP Proliant servers:
The HP ProLiant server sizer for Microsoft Virtual Server is an automated tool that assists the user in selecting the server environment necessary to support Microsoft Virtual Server. The sizing information and algorithms have been developed using testing and performance data on HP ProLiant servers.
The HP ProLiant server sizer for Microsoft Virtual Server 2005 R2 is an automated tool that assists the user in selecting appropriate HP ProLiant servers. This tool provides a quick and consistent methodology to determine a “best-fit” server for your individual needs.
The sizing information and algorithms have been developed using testing and performance data on HP ProLiant servers running the Microsoft Virtual Server 2005 R2 Enterprise Edition.
You can use it here.