Chegs view is not what I have seen and read.
Vista is going to be launched in a number of flavours. (Up to seven has been mooted)
If you want the all singing all dancing mega version 'Aero Glass' then yes you will require a hi spec machine.
However for the average user lesser versions will offer virtually all of the functionality.
I am beta testing Vista at the moment and my machine whilst well speced is not by any means top of the range.
MS are not going to spend millions (if not billions) of dollars's producing Vista if no one can use it on their current machines.