This is one of those complicated questions with many answers. Do you need to buy a full version, no. Just buy the cheaper upgrade or else oem install. As far as can you do an in place upgrade from vista, that is it copies all settings and programs over to windows 7 from vista, then yes. AS long as your not moving from 32bit to 64bit and your going to an equal or greater then version. Very complicated question.
I went ahead and brought the Full Install (Promotional Version) because it was priced lower than the cheapest Upgrade. Now I need to know if I will have to do a clean install or should I Partition for a Dual Boot?