SQL Server 2008

If you need to install SQL Server 2008, unfortunately you must uninstall all components of SQL Server 2005 if you already have them.

For other versions comparison check this: http://msdn.microsoft.com/en-us/library/ms143694.aspx

The upgrade path: http://msdn.microsoft.com/en-us/library/ms143393.aspx

SQL Server 2008 Express Edition with Management Studio: http://geekswithblogs.net/campuskoder/archive/2008/10/08/125727.aspx