The sole purpose of this extra pin is to make certain that you only use Xeon MP processors in 4-way configurations, which is a shame since the Xeon DP processors ship at much higher clock speeds but we'll get to that later. http://www.anandtech.com/IT/sh...
What does this extra one pin do? Well, nothing. The 604th pin is electrically inert, and simply is there to prevent you from installing a new 533 MHz FSB Xeon in a 400 MHz FSB motherboard. http://www.gamepc.com/labs/vie...
Hvorfor skulle soklen diktere fsb? Det er da chipsettet.. Man stoppede bare med at lave chipset til 603 og derfor kom den aldrig med op i hastighed... F.eks var Socket A først 100fsb med Thunderb., så 133fsb med Palomino og til sidst 166fsb med Barton..