Keys are interchangeable between like verisions. If you know someone with a 32bit oem disc you could use it with the key you get from a oem 64 bit verision. It works the other way around too.
What you plan on doing will work too but I would hold off all together on getting windows 7 untill your ready to go x64. There are not a lot of big differences between vista and win7 that would have me going through the trouble your describing.