I do not understand what the problem is with Office 32 bit running on a 64 bit system. It works fine. Office 2010 has just been released in 64 bit format. If you wanted to go that route it would mean a new purchase of Office. The gain would be negligible. Have a look at the following sites for more info.
The main reason is app compatibility, which is why 32-bit Office 2010 is recommended even on 64-bit Windows. There are very few if any 64-bit add-ins for Office. For a more thorough discussion on the difference between 32-bit and 64-bit Office, please take a look at these docs:
http://technet.microsoft.com/en-us/library/ee681792(office.14).aspx
http://msdn.microsoft.com/en-us/library/ee691831(office.14).aspx