Windows 7 Professional 32-bit vs. 64-bit

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.

Why have a 64-bit OS if you're only (ever) going to be running 32-bit programs on it?

Because the OS can address more than 4 GB of RAM. The programs individually may not be able to address more than 4 GB of RAM but memory usage does mount up when a lot of programs are open. I don't know how much of a power user your customer is but there are some advantages of having more memory or the ability to add more memory.

If the programs can all run on 64 bit then go for it. Its more or less becoming the standard version of Windows these days. Microsoft recommended the 32 bit version of Office when it came out, not because the 64 bit version of Office didn't work aswell but rather non-Microsoft 3rd party addins didn't work in Office 2010 when it was released. For me it took some time for both Mendeley Desktop and Mathtype to release a 64 bit compatible addin - I now use the 64 bit version.

I would recommend sticking to the 64 bit unless a program doesn't run on 64 bit but most programs will.

Most programs these days will run on both 32 and 64 bit Windows. I don't know how long it will be until programs are written 64 bit only.

The end of 32-bit OS's is in sight.  Server OS's have already dumped 32-bit ... Servers 2008R2 and 2012 are 64-bit only.  It was thought early on that Windows 8 would also be 64-bit only, but I think the fact that they tried to make it compatible with computers that are capable of running Windows 7 and the mobile platforms put that on the backburner for now, but there is a very good chance that the next [major] Windows release may not even come in a 32-bit version.  I also heard that Microsoft told developers with the release of Windows 7 that they had 6 years to port their stuff to 64-bit (puttting that around 2015 - historically, the projected date of Windows 9).