As long as you have a legal Vista license, you can buy the upgrade version and do a clean install. (Note: by upgrade I do NOT mean the one that lets you upgrade from one version of Windows 7 to another version of Windows 7).
If you owned a valid Vista license that you're no longer using, then you're OK to buy the the upgrade version. As I understand it the upgrade version will install if it detects any previous OS on the system, including a version of Windows 7.
I just followed the same instruction mentioned by sminlal from that Paul Thurrott's Winsupersite, back in december while installing Windows 7 in my dell studio 1450 using upgrade disc.
Worked like a charm! Activated without any problem in the first time itself!