I've spoken with some folks that are planning to move their enterprise from SQL Server 2000 to 2008. The first question they have is "Is that supported?" - the answer is yes! There is a lot of documentation you can find on the new version, and of course Books Online has a ton of information on upgrading.
But you can also take a peek at some of the early sites back in 2004/2005 on upgrades from SQL Server 2000 to 2005. Several of the new points there still hold today, such as the change from SQL Server Data Transformation Services (DTS) to SQL Server Integration Services (SSIS). The best approach I've found so far is to read the 2008 documentation first, and then skim through some of the shorter articles on the web on upgrading to 2005. With all that knowledge and a little planning, you'll be all set.