The Microsoft public information page has most of everything you need for your upgrade planning, testing, strategy and techncial information here. My personal recommendations are to check-out Scalability Expert’s Assistant tool that is linked at the bottom of that Microsoft page. And be sure that you also run the Microsoft upgrade Advisor tool, toward the upper portion of the info web page.
The technical reference guide for upgrades on that page is invaluable and I use it all the time when planning customer upgrades for SQL Server. Download it and keep it handy as a way to reference specific parts of your upgrade project. You probably won’t get to read the entire document. It is almost 500 pages long!
Another set of partners, Dell & Principled Technologies, put together a SQL Server 2000 to 2008 migration guide for those of you out there needing to upgrade your older SQL Server 2000 databases. This PDF has a section on migrating database logins, a popular topic in SQL Server migration-land. It also has information on discontinued features and mentions several best practices.
If you are moving from SQL Server 2000 to SQL Server 2005 or SQL Server 2008, remember that DTS was re-written as SSIS. Check out this handy posting on Search SQL Server by Joe Toscano for how to approach migrating from DTS to SSIS. And be sure to utilize the SQL Server DTS Migration Wizard to help with migrations to SSIS.