Now that Office also comes in a 64bit install, where in the registry do you find out if the version of Office installed is 32bit or 64bit?
I found this approach:
If HKLM\Software\WOW6432Node exists then Windows is 64-bit.
If HKLM\Software\WOW6432Node\Microsoft\Office exists, then Office is 32-bit.
If HKLM\Software\WOW6432Node\Microsoft\Office does not exist, but HKLM\Software\Microsoft\Office does exist, then Office is 64-bit.
If HKLM\Software\WOW6432Node does not exist, then Windows and Office are 32-bit.
Source: Technet Forums