This is certainly a complex question.How much memory should an OS like Vista require? How did you arrive at that number or range? What is it that drives your perception?
My from-the-gut answer would be 256MB. What drives this, is the fact that I have a computer with 256MB that I use productively with Win2K. It pains me to think I'm going to be losing support for this machine at some point. Obviously this is quite subjective, but my opinion nonetheless.
Whether or not this is feasible is entirely another question. Thinking wishfully, it would be possible to have the OS scale from requiring very little memory, to quite a lot, as a function of what services/features one is using. I suppose to some extent this is already true though...