If you are planning to upgrade from Sharepoint 2007 to Sharepoint 2010, Microsoft has released a free guide that could prove very useful.
Here’s what the guide contains (excerpt taken from the document)
About the upgrade process (SharePoint Server 2010) - Learn about what's new in upgrade and how the upgrade process works.
Plan and prepare for upgrade (SharePoint Server 2010) - Determine which approach you should take to upgrade to SharePoint Server 2010 and plan your upgrade process.
Testing and troubleshooting upgrade (SharePoint Server 2010) - Learn how to test your upgrade process ahead of time to understand what issues you might face in your actual upgrade, and determine the time and space you will need for upgrade. Also, learn how to troubleshoot issues that come up during the actual upgrade.
Perform pre-upgrade steps (SharePoint Server 2010) - Find out what steps you need to take before upgrading, including information about how to run the pre-upgrade checker.
Perform an in-place upgrade (SharePoint Server 2010) - Follow the steps in this section if you are upgrading in-place to SharePoint Server 2010. When you upgrade in-place, you install SharePoint Server 2010 on the same hardware, and then upgrade the content and settings on the server or server farm as part of a single process.
Perform a database attach upgrade to SharePoint Server 2010 - Follow the steps in this section if you are using the database attach upgrade method to upgrade to SharePoint Server 2010. When you use the database attach upgrade method, you upgrade the content for the environment on a separate farm.
Perform post-upgrade steps (SharePoint Server 2010) - Find out how to tell whether upgrade was completed successfully and what steps you need to perform after the upgrade to get your environment ready for users again.
Upgrade from other versions or other products - Articles in this section contain information about how to upgrade to Microsoft SharePoint Server 2010 from related SharePoint products.
Migrate from forms-based authentication to claims-based authentication (SharePoint Server 2010) - This article provides guidance to help you migrate existing Microsoft Office SharePoint Server 2007 Web applications, which were configured to use forms-based authentication, to work in a Microsoft SharePoint Server 2010 environment as claims-based Web applications.
Migrate from classic-mode to claims-based authentication (SharePoint Server 2010) - The procedures in this article provide guidance to help you migrate existing Microsoft SharePoint Server 2010 Web applications, which were configured to use classic-mode authentication, to use claims-based authentication.