With Windows 7, you’ve the option of running 64-bit Windows on your 64-bit PC, and now with Office 2010 you’ve that same choice. As 64-bit processors and operating systems are becoming standard for systems ranging from servers to desktop computers, 64-bit Office will be able to take advantage of everything that 64-bit systems have to offer. “On 64-bit Windows, you can install a lot more physical memory. While we work hard to minimize the amount of memory our applications use (Office 2010 has the same minimum memory requirements as Office 2007), this ability to access more memory means that 32-bit Office applications on 64-bit Windows will be able to open, edit, present documents, and switch among applications much faster than on 32-bit Windows. This's especially true if you run other memory-intensive applications alongside Office. Of course, your ultimate performance will be determined by relationship between amount of virtual memory being actively used and the amount of actual memory installed on your system,” noted Ted Way.
More info: Understanding 64-Bit Office