From what I gathered, Microsoft "patched" bad mirroring on SP3, or one of standalone patches.
You can use WAY more than 2GB VRAM + RAM combined, however like you wrote, it's virtual memory thing along with "3GB flag" (IIRC).

Basicly : You will be fine on crashes side, as long as game was made aware of this.
Game that doesn't like 3GB flag is Skyrim (with high res texture packs and other mods), and one that doesn't care about it is Crysis. Still, you need quite a lot of resolution/detail to go over 2GB mark (and GPU power to actually make it playable). It's not that surprising nobody tested this (since there are only handfull of cards that support WinXP and have over 3GB of VRAM).

Here's Crysis test under 3GB RAM + 3GB VRAM :
LINK

And working Titan under WinXP :

Can you also post the crysis game consumed how much memory in the task manager or other program that displays how much crysis game is using the memory?
Or can you confirm if the crysis game used MB in other memory monitoring program?

Your GPU-Z shows vram 762MB used on this titan X card on this 32bit OS but this also 1838MB of the memory allocation but if I take away 762mb from 1838mb, I get 1076mb used for crysis. What does task manager memory says for the crysis? Windows does not report the vram in the memory monitor.

All the posters on every forums cautioned against using /3GB switch.

Problem is every forums also does not mention about application address allocation which is 2GB per application on 32 bit XP, also does not mention how video card's vram (used) is reflected in that 2GB allocation as well due to original programming design directx 9 and eariler, directx 10 and later solves this. Microsoft vaguely mentions this. I can count less than few fingers on one hand who mentioned these correctly.

Minimum is 256MB allocated, if the video card's vram is larger than this, vram used will grow into the 2GB application allocation, and if game/application is written poorly, may crash or stutters due to running out for the game needed to run properly if the vram get used so much. More seen often on 1GB and greater GPUs.

The problem is we also need directx 9 so we can game on older games that uses 9 and earlier.

Also many posters says not to task switch as the memory monitor program task manager memory used monitor would not show this except for some GPU memory monitors like GPU-Z recording chart works right but this only shows the VRAM used but you need the other half is application's memory consumed .

What I mean is game used part of the 2GB application allocation and vram used added too due to XP 32bit can only address 3.5-3.2GB on typical configurations but it is not usual to see *less* than this even 2.x GB range depending on add on cards installed. Means 4GB video card is wasted on 32bit OS unless move on to 64bit OS and use directx 10 or later.

Video card vram is not exclusive of application addressing range. This means if the 700MB vram used on a 1GB video card means this is now part of 2GB game's which also let say is using another 590MB.

This add both together, now have 1290MB used of the windows XP's 32bit allocated 2GB for game or application means 710MB left.

This why I need confirmation from others who know about this especially windows 32bit OS programmers's input on this.

Please refer to the techpowerup forum link I provided.

Cheers, pentiumspeed

Doesn't recent GPU-z versions, monitor system RAM use under "System memory used" ?

Either way :

^Picture from video I linked before.
You can see both VRAM and RAM usage live during gameplay via RTSS.

I didn't checked how much memory Crysis process itself uses, however at reported 2,7GB VRAM usage it's pretty much irrelevant if you think about 2GB limit 😁
Settings used :
3840x2400 + 8xQ MSAA + High preset details under Crysis (1).