Apps don't have to be 64-bit to run on a 64-bit app ... 32-bit apps work just fine in a 64-bit OS.  Like you said, Microsoft introduced the first 64-bit version of Office in Office 2010, but they still recommend running the 32-bit version on a 64-bit OS. There are only a handful of reasons that a 32-bit app won't run correctly on W7x64.  For example, 2003 remote server management tools will not run on 64-bit.  There are more 32-bit apps that won't run in Windows 7 because it is Windows 7 than 32-bit apps that won't run in 64-bit Windows because it is 64-bit.  Having run them on Vista means that they will "probably" also work on Window 7, as Vista has many of the same requirements and restrictions as Windows 7. I would look here for the specific apps and versions that he will plan to run: www.microsoft.com/.../Home "Why have a 64-bit OS if you're only (ever) going to be running 32-bit programs on it?" The best reason?  Right now, he only has 32-bit applications.  Is he going to upgrade to 64-bit applications all at once?  Probably not - one at a time, probably as needed.  Software companies may not feel they need to write for the 64-bit platform in the near future, so 64-bit apps may not ever be available for a product, and/or not available when he and his other apps are ready for the jump to 64-bit, so having 64-bit already installed just lets him upgrade as each is available and he is ready to upgrade that app.  Keeping in mind that there is NO in-place upgrade path from 32-bit to 64-bit, a full reinstall will be required to move to 64-bit - potentially a disruptive amount of downtime, re-adusting settings and preferences, updates, reinstalling programs, etc.  Also, say he begins maxing out his RAM (max of around 3.5GB for 32-bit) ... a memory upgrade will require a reinstall of 64-bit before any more memory will be usable.  With 64-bit, he can throw all the memory his system will support at it with only a reboot to install the memory in the system. My thought:  If it will work on Windows 7x64, and this is a mid-long term solution, there is no reason not to use it.
I will soon be configuring a new Dell unit with an i7 CPU, SSD & a good NVIDIA video card and my customer has specifically requested that the Windows 7 Professional 32-bit OS be installed on it due to the fact that he will be running only older legacy apps on there (with the exception of Office 2013, but which I will still be installing the 32-bit version of).  Other than the fact that it will only address 4GB of RAM (which is no big deal to him), is there any other compelling reason not to install it instead of the 64-bit version? I'm wondering if the overall performance of the OS will be affected based on how it interacts with the CPU or some-such?  (I'm speaking of the 64-bit version here).  At this point in time there are few, if any 64-bit programs available that the average home user could (or would) utilize; he doesn't do any video or even photo editing or any other high-end, processor-intensive activities on the computer. Plus, this would save having to install dual versions of some of the other routine apps, such as Java & IE 10, etc. If & when he ever goes "up" to Windows 8 or (at this point) Windows "Blue" then it might make some sense, but he just doesn't use any new and / or "high-powered" programs which would benefit from a 64-bit version of Windows, as far as I can tell. I'm open for any compelling reasons to go 64-bit under the circumstances (other than the RAM issue, which is actually a non-issue) at this point in time...
Windows 7 Professional 32-bit vs. 64-bit