Wide Open VistasMicrosoft Windows Vista Enhanced Professional Edition 1.0 SP2 (2007 hopefully) equals OS X 10.1 (2001) except for security, ease of use, size, price and speed.
Amazingly, OS X with every new version (3) has become faster and faster and can run on smaller and smaller machines. Contrast that with the Cray-like computer that will be needed to run Vista.
Vista is just like OS X except for adding restrictive DRM, undocumented backdoors, propietary networking protocols, backwards compatibility, reduced documentation, administrative overhead. Oh, I give up. People either know this already or don't wish to know. My clients with mixed environments always make sure there is a machine running Windows 2000 Professional so that they can always be sure of at least one mission critical Windows workstation.