3GB switch will not function on WinXP w/4GB RAM installed

No need to use any switch. XP will see up to 3.5GB. Depending on the video ram.

The /3GB switch has nothing to do with making Windows report, utilize, or address more RAM. The /3GB switch is used to give applications more virtual address space, and only applications that explicitly request it (98% of 32-bit applications aren't coded to support this).

Remove the /3GB switch, it can only create problems unless you understand exactly what it does and when to use it (and you don't). The limitation here is the Intel 945 chipset, which no amount of boot switches can overcome or work-around.

I used the term 'coded' in an over-simplified (though technically inaccurate) sense because I figured that someone who already did not understand what the /3GB switch does probably would get stuck on "set LARGE_ADDRESS_AWARE flag in the image header".

I would disagree with your characterization of how common or prevalent are 32-bit applications with large address support. It is uncommon in the extreme outside of enterprise, database, engineering, scientific, and professional applications. Even within these segments, it is not the norm. I don't know of any that install by default with large address support, and most developers state it is for testing purposes only or not supported in a production environment.

Even Microsoft does not support /3GB (4G Tuning) in a production environment on Windows XP Professional. If you're a developer having problems testing your applications against it, they might take a look. But end-users are on their own.

Given the potential implications for kernel-mode stuff, the risk of application failures because so few 32-bit apps are tested or designed for it, the exceptionally narrow scope where it is indicated or might even prove beneficial, there is little point to indulging its potential merit on a PC running 32-bit consumer OS and applications. One in every ten thousand might realize some benefit or get it working with no apparent adverse consequences.

Really? I would appreciate if you could provide some examples. And we are talking about 32-bit applications and 32-bit OS. Installers that check for 64-bit OS, perhaps (though I doubt all that many more are), since 32-bit applications claiming compatibility under 64-bit OS are almost guaranteed to have been tested in large addressing modes. If they weren't, the developer's testing for compatibility with 64-bit OS was fundamentally incomplete.

Other than crashing the application, the operating system, the risks of data loss that result from crashing applications and operating systems, and failing to boot as the OP clearly reported. Except those things, it won't hurt anything.

That's great. Now if we could only find a discussion that is remotely relevant to 64-bit OS, your repeated mention of it might actually seem much less pointless there.