Yes Vista and XP are very close to the same Memory usage now. As for your quote that is only if you try to install Vista and XP side by side on the same drive. That isnt recommended at all. It does corrupt your XP install that way.
The best way to install Vista is by having a 20GB (or so) partition to install it on. That is what i do and i have gone back and forth between Vista and XP flawlessly. I dont have the folder porblems nor do i have any crashes.