Hi,

I am developing an application which needs to backup data between SQL
Express 2005 and SQL Server 2008. My client runs with an installation
of SQL express 2005 and needs to periodically back up data to a server
database running on SQL Server 2008. The client db also receives some
new data from server and needs to update itself. The question is HOW.

You can create a backup on SQL Server 2005 and restore on SQL Server 2008. But you cannot restore SQL Server 2008 backup
to SQL Server 2005 (as the format is not backward compatible). The standard technique is to script table structure (if
there are changes) and to export the data, then re-import on SQL Server 2005. You can also consider replication if only
specific changes need to be replicated.

--
Plamen Ratchev
http://www.SQLStudio.com