You may have noticed that many of the computer manufacturers such as HP, Compaq or Dell are offering many ready-to-ship systems with a 64-bit Windows 7 version as the standard OS. You're not seeing a mis-print, the 64-bit Windows versions are becoming more popular for their ability to utilize memory capacities above 4GB, where the more mainstream, 32-bit versions are limited to under 4GB.
Although the more powerful 64-bit Windows versions still have a somewhat limited compatibility with some older hardware and software, many power-users are willing to sacrifice that for higher memory capacity. Corsair Memory's official blog entry "WINDOWS 7 AND THE BENEFITS OF ADDING MORE RAM" by Gareth Ogden on October 20th, 2009 gives you a good idea of how the 64-bit version of Windows 7 takes full advantage of running 8GB of RAM versus 4GB.
Gareth Ogden at Corsair states:
"The launch of the new Windows 7 operating system from Microsoft presents budding enthusiasts and early adopters with some exciting opportunities to improve the way in which they use their PCs. Windows 7 has received widespread critical acclaim, and for good reason, because it offers some compelling advantages compared to previous Microsoft operating systems. And by upgrading your existing PC, or building a new Window 7-based PC, you can maximise these benefits.
One such benefit is that Windows 7 is highly efficient at making use of high-density memory kits, by which I mean kits of 8GB or larger. Combined with Windows 7’s slick new user interface, which makes it simple to use and manage multiple open applications, Windows 7 is the ideal operating system to really take advantage of the 64-bit memory addressing and multi-tasking capabilities of modern processors...
...With 8GB or more of RAM there is basically enough space in which to store all the data the operating system needs, whereas with 4GB or less you often experience performance ‘blips’ or stutters that occur as a result of the system accessing data in the paging file on the storage drive.
The best way to experience how adding memory improves the responsiveness/feel of the computer is to actually use 8GB or more of memory for a few weeks, and then drop back to 4GB. As Joni Mitchell famously said (though probably not in relation to DDR memory), “you don’t know what you’ve got ‘till its gone.” and this is very true of more RAM. With 8GB or more you simply get used to the smoothness and responsiveness of the system, and the ability to leave all your applications running. The latter aspect is where Windows 7 really helps because, as mentioned earlier, the interface intelligently manages the different windows and applications, making it easy to navigate."
Windows 32-bit versions are a good choice for the average user with older hardware and/or software, or someone who only uses one or two windows at a time. Those who consistently bog down their computer with multiple windows, programs and memory intensive processes, the 64-bit versions will serve them well.