Is there a benefit of using Server 2008 over 2003 for SQL Server? Specifically 2005?
In general use Windows Server 2008 (R2 if possible) when you can. You're benefiting from the additional years of tweaks and fixes that Microsoft has made to the operating system. You're also going to have support from Microsoft further into the future, since Windows Server 2003 is going to become unsupported before Windows Server 2008 does.