"Why should you upgrade to Vista?"
This week's Technet flash is a special with the above title. One of the sections concerns performance - "There is another perception that Windows Vista underperforms in every area of the system when compared to Windows XP. "
It's worth looking at it and watching the video. This is a classic of how you can prove anything you want with statistics. What it's supposed to show is that Vista copies big files much faster than XP. What it actually shows is that if you have Vista machines on your network, they will trash the performance of your XP machines!