Kent pointed me to this article by Kenton Gardinier in Windows Server System Magazine (previously known as .NET Magazine).  It walks you through ensuring adequate capacity for your SharePoint Portal Server installation, while at the same time reducing risk, and easing administration and maintenance. Looks like there's some good advice here.

You can alleviate many issues in the long run and ensure overall proper sizing and design by beginning SPS design with thorough storage-capacity planning. Capacity planning is the process of determining and understanding technical requirements, such as resource capabilities, and matching them with business requirements. Mapping business objectives to technical requirements in the context of SPS storage can be one of the most challenging aspects of building an SPS document- or knowledge-management solution.