I see that you are in England, so I will assume that MS treats you the same as they treat us in USA. I bought an upgrade from Vista Home Prem to Win 7 Home Premium. I received 2 DVD one for 32 the other for 64 bit. There is one Key which is good for either the 32 or the 64. Of course, it can be used only once. Bottom line, based upon what I know, in the USA, you will not have a problem.
Incidentally, Win 7 is really great. I had Vista for a year and this is so much nicer. Especially since there is no learning curve.