In a production environment where system and application functionality is in the "mission critical" category, you don't just change platforms on a whim. This is particularly true if the software in use is specific to the line of business the machine/s is/are used for. Field-specific software solutions tend to be the slowest to adopt new technologies; just look at how many threads we've seen about Amicus products having problems on Windows 7. On the flip side, hardcore multimedia production software are most likely to be quickest to make use of hyperthreading capabilities and such. So, it depends on the field.
Set up a test platform for the purpose of examining the feasibility of upgrading to 64-bit. If existing software in use does not play nice with 64-bit Windows, then consider upgrading the software. If this is unacceptable or there is no 64-bit compatible version, I'd suggest you recommend against a platform upgrade. As a last resort you might be able to run the old 32-bit junk in a VM.
If your customer is set on upgrading despite your presumable recommendation to the contrary, then it's his money to blow. My uncle who fixes cars for a living makes a nice living fixing cars previously fixed by so-called "do-it-yourself" guys.
I'm just saying, there's a different way one ought to approach upgrades to a business workstation as opposed to a home desktop.
