1) Something is amiss. The only multi pc windows-7 package that I know of is the family pack which allows installation on three pc's.
I suspect you got a family pack that has already been used once and has two remaining installs. I think there is fraud here.
2) I think all of the windows 7 dvd's are exactly the same, and they include components for all versions. Which version(home, pro, ultimate) you install and how(clean vs upgrade) is validated by the product key. There is a 32 bit dvd and a 64 bit dvd.
3) The product key is insensitive to the 32 bit or 64 bit flavor,allowing you to change either way.