Capacity Planner 2007 for SharePoint 2007, Exchange 2007, System Center
Are you designing a architecture for the Sharepoint , Exchange - then these tools
will be useful. This is only for Architects.
Capacity Planner 2007 :http://technet.microsoft.com/en-us/sccp/bb969059.aspx -
You can find the tools here. 
First install the SCCP
2007 the install the Sharepoint capacity planner as add on.
To access the System Center Capacity Planner 2007 download, click here.
The SharePoint Capacity Planning Tool is a software extension of the System Center
Capacity Planner 2007 (SCCP2007).
Then install Sharepoint Capacity tool - Download here http://go.microsoft.com/fwlink/?LinkID=93029.
Now start using the Sharepoint Capacity Planning tool.
input the configurations of the Hardware
Input the redundancy required in the WEB and SQL server ends.
Once done with the inputs run the Simulation where you get the results
Thanks
E.Ravi
Read the complete post at http://www.eravi.net/blog/2009/05/19/CapacityPlanner2007ForSharePoint2007Exchange2007SystemCenter.aspx