One of the most common planning failures – and cause of future headaches when system limitations are reached – is the failure to conduct proper capacity planning before, during, and following 
your SharePoint migration. Your SharePoint environment is a living, breathing life form, with end users and admins constantly making changes. Prior to any migration, you need to assess the current system, think about your growth and performance requirements, and plan for your future operational needs.
You must understand your current environment, including:
- Site and user data (number of users, sites, site collections, overall activity)
- Storage and SQL Server requirements
- Geographical needs of your organization (across sites, collections and farms, understanding their usage patterns)
- Line of business application integrations, use of various services in 2007, and plans for 2010
- Hardware
- Topology
- Performance requirements
- Search architecture
Additionally, you need to think about your future needs, including user growth, estimates on site creation, estimates on database growth, and security and search needs. Based on existing and projected usage and the above environmental factors, you will also need to map out your security requirements, scalability needs, disaster recovery plans, and business continuity strategy. To help you in this process, Microsoft has created a site specifically for capacity management of SharePoint 2010, providing guidance and relevant links.
For additional reading, Gurmeet Singh did a great job of collecting all of the relevant content you may want to familiarize yourself with for moving to the 2010 platform:
Finally, and in addition to the award-winning ControlPoint administration platform for SharePoint, which can help you monitor and manage your SharePoint 2007 and 2010 environments, Axceler provides a free tool called PinPoint that helps you understand what users are doing on your environment, including largest sites, most active, largest items, and most active items. PinPoint will help you develop your governance plan, and better prepare you for your migration to 2010.