1) Can we attach SQL 2005 DB .mdf file in SQL Server 2008 ?
Yes, you can
2) Can we restore SQL 2005 User Databases in SQL 2008 ?
Again, no problem, but you need to change the compatibility level manually.
and 3) Can we restore SQL 2005 System Databses (Master,MSDB..etc) into SQL Server 2008 ?
Don't even think about it.
While it's theoratically possible, you will end up in all kind of problems. Script out your Logins, Jobs, Alerts etc. or use SSIS to transfer these to your new machine.
[font="Verdana"]Markus Bohse[/font]