When starting out with a new SCOM deployment or sometimes after running some time you might find yourself asking for an estimation of what resources are needed to run SCOM.
To start with I would suggest using the SCOM design guide:
http://technet.microsoft.com/en-us/library/dd789005.aspx
Next check out the Operations Manager 2007 R2 sizing helper:
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=23016
These will help in giving an idea of what you will be up against for SCOM deployments of different sizes. I would suggest always using some margin with this. Reason is because you might find yourself adding more and more to the monitoring. For instance cross platform monitoring, network monitoring, hardware and storage and so on can give a performance hit on the SCOM servers and increase the size of the databases.
