With SQL 2005 support expiring shortly, the move is on to switch to SQL 2008 R2 (why would you switch to just SQL 2008?)
Running upgrade advisor before the upgrade is a must. However, some of the warnings could safely be ignored rather than changing a potentially large codebase.
I have seen many people asking how shall they move towards upgrading their SQL Server 2000 Database to SQL Server 2008. I thought of putting together an article based on my experience where I have migrated many servers from SQL Server 2000 to 2008. This particular Article is about analyzing incompatible code which is a MUST step in my view. At end of this article, I have compiled list of various errors/warning that you may come across after running analysis tool.