When Microsoft Office 2010 debuts in June, consumers will be able to choose for the first time between 32-bit and 64-bit versions of the suite. Microsoft's own Office blog offers a quick primer on which version you might want to select.
The full post is an interesting road read, but the outcome is a little surprising: despite believing that most users will shift to 64-bit operating systems in the near future, Microsoft is recommending that most users run the 32-bit version even if their OS is 64-bit (and will do this unless you explicitly hunt down the 64-bit setup file on the install DVD). The reason? While the 64-bit release can handle larger files and might be useful for Excel addicts, it also has some compatibility issues:
The extra memory capacity comes at the cost of some compatibility with existing extensions to Office, such as 32-bit versions of ActiveX Controls and some 3rd party add-ins, in addition to 32-bit versions of programs that interface directly with Office. New versions of these extensions will need to be obtained, and it will take some time for 64-bit compatible extensions to be made available.
For these reasons, we recommend running 32-bit Office 2010 even on 64-bit Windows operating systems for better compatibility. On 64-bit Windows, more applications and documents may be opened at once, and switching among them will be faster because the machine can have more physical memory for the processes to share. When the 64-bit ecosystem for Office is more mature, you’ll be able to easily migrate to 64-bit Office!
As that comment suggests, you can switch from 32-bit to 64-bit, though you need to uninstall Office first.
Understanding 64-Bit Office [Microsoft Office 2010 Engineering]