Here is an interesting chart (courtesy of
California Data Center Design Group) showing space usable for equipment racks at various power densities. At 500 W/sq ft, power and cooling (M&E) equipment space exceeds server equipment space by 3:1. Since utility room space is less expensive to build out than server room space, higher dentisy = lower building costs.
http://www.andrew.cmu.edu/user/kb13/Density.gif
Ken