Have an existing SQL Server instance that you need to upgrade? There are many different editions out there such as Express, Web, Workgroup, Standard, and Enterprise. This gets compounded by the different releases (SQL Server 2000/2005/2008) and different OS platforms (32-bit vs 64-bit).
For a quick check at the upgrade path of you SQL Server instance, you can find it on Microsoft’s website: http://msdn.microsoft.com/en-us/library/ms143393.aspx.
I recently had to look this up for a client who had an Evaluation copy of SQL running. To my surprise, there is no upgrade support to a full edition from an Evaluation copy.
Also, just in case, if you want to compare features of each edition of SQL Server 2008, you can go here. They’ve conveniently grouped features so you can zoom in at the feature you’re interested in.