http://download.microsoft.com/download/4/b/4/4b455e48-72c4-4a04-b9a5-892fd497087a/TollyResults.pdf

This white paper of a third-party benchmark study done by The Tolly Group compares network throughput and time-to-completion of several tasks when using Windows XP, Windows Vista, Windows Server 2003, and Windows Server 2008 (now in beta testing).