Except that upgrade copies of Windows 7 are now hard to find. I'm told by a shop-manager that Microsoft has ordered all unsold upgrade discs to be returned. I looked high & low for an upgrade version last week, no chance.
Non-upgrade versions are still available though, mostly the more expensive Pro & Ultimate editions.
This is indeed true! I work in retail and they requested all of them (upgrade & retail) be pulled on October 27th which is the day after Windows 8 launched.
As for installing XP and then 7, you should not have to. You will be installing a clean copy of Windows (whichever version) and it in essence will see it as a clean install regardless.
Like geofelt mentioned there are issues with doing this however so I would recommend viewing the site they previously mentioned.