System Center Capacity Planner and Configuration Manager 2007 = False
At the moment there are no plans to use the System Center Capacity Planner for Configuration Manager, instead you will have to use the online documentation. Some good reference links when planning our site will be found below.
Planning and Deploying the Server Infrastructure for Configuration Manager 2007
Configuration Manager Single Site Planning and Deployment
Configuration Manager Site Capacity Planning
Configuration Manager Site System Planning
You can still use the SMS 2003 Capacity Planner for some usage but you will have to count with that configuration Manager will require more hardware and bandwidth than SMS 2003. Of course depending on what new functionality you will be using in your environment for Configuration Manager 2007. But at least it will point you in a rough direction.