I can't believe that MS would require at least a DX9 card just to run the OS
If you want to run it with all of Aero's effects then, yes, it will require a DX9 class card (and, come on -- that dates back to the ATI Radeon 9700 and GeForce4 series of cards). But you can certainly run Vista without all of the Aero eye candy -- although eventually I bet you'll not be able to use some applications.