Windows AERO theme basically turns your desktop into a full 3D application. Without a videocard that can meet the needs to this new desktop, vista will default back to 2D.
They have been testing Vista Beta 1 with new ATI X1900XT's & X850XT's with ATI vista Beta drivers and bombarding the desktop with 3D windows with full effects enable has been flawless.
The bottomline is, MS hasn't released the (Official Requirments) yet.
It's too early to speculate on this and that. All i'm trying to say is, research vista, check it out google it... but don't think about hardware upgrades now solely based on vista and DX10.