The problem with Vista is not that it is a hog, or that it is worse than XP. Relatively speaking, Vista is as big a system hog as XP, and in terms of features, it's probably a bit better than XP. However, jsut a bit better really doesn't cut it, if you want to get people to change to a new, quite expensive OS.
XP was a success because it was significantly more stable and secure than 98SE, which made it a worthy upgrade for a lot of people. I still use XP because it never, ever crashes and I've never had any security issues with it. Why would I spend an ungodly amount of money on Vista if I'm already happy with my current OS?