>It's all a matter of timing. If you really need a computer NOW then Vista is fine. If you can wait for Win. 7 then just buy your computer close to the launch of Win 7. They you'll get both OSs. Vista and a free upgrade to Win 7 ;-)
Doesn't the license agreement for the upgrade usually state that the new version replaces the old version (i.e. you only have a license to use one or the other)? Obviously this would preclude you from installing the upgrade on a different computer. What I don't know is if you'd be in violation of licensing agreement if you use two separate harddisk drives -- one with the older version, and one with the upgrade (but only one hooked up at any time).