Walt Mossberg on the differences between the many versions of Windows 7, and how to upgrade to them:
However, there’s another complication. For each of the three main consumer versions of Windows 7, there are actually two editions. One is meant for PCs with standard processors, called 32-bit processors, and the other for PCs that sport newer processors called 64-bit processors. The 32-bit version of Windows can recognize only 3 gigabytes of memory, but the 64-bit version can use much, much more. For most average users, 3 gigabytes is plenty, but some consumers have 64-bit Vista machines, which can move faster when lots of programs are being used at once, or when doing tasks like playing back high-definition video.
The problem is that you cannot directly upgrade 32-bit Vista to 64-bit Windows 7, or vice versa. So that adds another layer of complexity to the upgrade process.