While Activity Monitor in SQL 2008 does a good job depicting waiting tasks, top N queries etc.; many DBAs are simply too familiar with the erstwhile SQL Server 2005 Performance Dashboard Reports. While officially Microsoft does not support the usage of these reports with SQL 2008, I found two links of interest which will help you get these reports up and running in 2008. But please keep in mind: this is AS-IS!
Link 1: http://blogs.msdn.com/b/vascov/archive/2008/09/30/using-performance-dashboard-with-sql-server-2008.aspx
Link 2: http://blogs.msdn.com/sqlrem/archive/2007/03/07/Performance-Dashboard-Reports-Now-Available.aspx
You have to perform BOTH the fixes to get the reports working. In Link #2, the specific place to look for is a comment which says ‘line number 3271’.
DO make a BACKUP of your recent_cpu.rdl file before you attempt the fix in Link #2.
[Update 26 Jun 2010] In addition, if you get an error related to an "overflow at runtime" then refer to the other fix in Link #2 above, the comment says 'Because DATEDIFF returns and int once'. Look for the fix from David.
Hope this is useful to some folks out there! If you liked the post, please do leave a comment and / or rate the post.