<p>Are there any performance benefits of using SQL Server 2008 over SQL Server 2005?</p>

Moving a single database from SQL Server 2005-2008 will not notice a difference really.  However, there are new tools and options available in SQL Server 2008 that you MIGHT be able to leverage to provider better performance later on in your application.
One item that comes to mind is filtered indexes.  Allowing to create an index on a subset of information.