Here’s an interesting table from CNET that reports on the 64-bit installed Windows base vis-v-vis 32-bit versions by OS for Windows 7, Vista, and XP:
It certainly looks like the ability to access and use RAM sizes in excess of 3.1-3.2 GB or so (about the best you can do with Windows 7 32-bit versions, even with 4 GB of RAM installed) has to be contributing to this change in the makeup of the Windows OS distribution when it comes to relative numbers of 32- and 64-bit versions for recent Windows OSes. With memory getting cheaper, increasing use of virtual machines, and a growing number of applications designed to take advantage of 64-bit data structures, file sizes, and so forth, there are also lots more reasons why it makes sense to buy a PC with 64-bit Windows pre-installed, or to switch from 32-bit Windows XP or Vista when upgrading to Windows 7.
As I look at my own PC population in-house, I see that except where 64-bit versions aren’t possible (as on Atom-based netbooks) or don’t make sense (as on older hardware with 4 GB or less of RAM installed), my own preference has been to upgrade to 64-bit Windows 7 on existing hardware. All of the systems I’ve purchased recently (except for an HP MediaSmart Server, for which 64-bit Windows is not available) in fact, have come with Windows 7 pre-installed as well.