Windows Vista Ultimate will be very very expensive, at US$399 a copy (that's about AU$524), according to Amazon.com's pre-order price and is scheduled to be released on 30th January 2007.
It' even more expensive to but Windows Vista from the good neighbour of the States. $C499 (about US$100 more than the US price).
In contrary, Mac OS X 10.4 "Tiger" is merely US$129. You can buy Mac OS X 10.4 through SillyDog701's link and support SillyDog701. Just a gentle reminder, don't steal Mac OS X!