So, you are considering upgrading to SQL Server 200x from a previous version. What are some of the key issues that you need to consider, or at least be made aware of, prior to upgrading?
Fortunately the heavy lifting of answering that question has already been done for you. It's available to you in the form of an Upgrade Technical Reference Guide. There's one available for upgrading to SQL Server 2008 and one for SQL Server 2005.
Of course, you'll still need to consider the information provided in these documents in light of your specific databases and servers, but at least these will provide a good jump start on the process.