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