Since the introduction of the 64 bit operating system, Microsoft released two different versions of Office 2010. A 32 bit version (which is used my most, and is compatible with many of the existing Office add-ons) and the 64 bit version of Office 2010 for use with the newer 64 bit operating systems.
You may have a 64 bit version of Windows (Windows 7 x64) but this does not always indicate that you have a 64 bit version of Office 2010 installed. If you are not sure which version you installed, the following article will describe how you can determine the version you are using.
Why use the 32 bit version of Office 2010 if I have a 64 bit operating system like Windows 7 x64?
The advantage of using the 32 bit version (the default installation, and recommended installation from Microsoft) is that your existing add-ons and templates that are designed specifically for 32 bit operating systems will (or should) continue to work with Office 2010.
The advantage of using the 64 bit version of Office 2010 is that you can utilize more memory and handle larger documents within Office 2010. So, unless you’re normally dealing and working with large files of 2GB or more, Office 2010 64 bit isn’t going to give you any advantage over the 32 bit version.
The following Microsoft Engineering Blog has some good information on this, http://blogs.technet.com/b/office2010/archive/2010/02/23/understanding-64-bit-office.aspx
NOTE: You can’t run mixed 32 bit Office and 64 bit Office 2010 components (side by side). If you install Office 2010 64 bit, all your Office 2010 software will be 64 bit (Outlook 2010, Excel 2010, Word 2010 …) The same is true for the 32 bit version. If you have the 64 bit of Outlook 2010 installed, you must uninstall Outlook 2010 64 bit and install the 32 bit version to install any other Microsoft Office 2010 32 bit applications
If you’re using any of the following software products, you’ll require the 32 bit version of Office 2010.
- Sending fax merges with the FaxTalk Fax Merge requires a 32 bit version of Microsoft Word 2010.
- WinFax PRO with a compatible operating system (Windows XP, 2003) will function with 32 bit Microsoft Office 2010.
- WinFax is not compatible with Outlook 2010 contacts.
- FaxTalk FaxCenter Pro 8, FaxTalk Multiline Server 8 and FaxTalk Messenger Pro 8 require the 32 bit version of Outlook 2010 to use contacts**
** FaxTalk FaxCenter Pro, FaxTalk Multiline Server and FaxTalk Messenger Pro can fax using 64 bit or 32 bit Office 2010 products.
How to determine the version of Microsoft Office 2010 installed
If you are not sure what version you have installed, you can check by clicking FILE from the Word ribbon, and then click HELP. 
A 64 bit or 32 bit version will be indicated in the dialog box as shown. (32 bit or 64 bit will appear after the version information)
