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Does anyone know of a foolproof way of determining whether, and how much, memory is being ‘stolen’ from main RAM for use by the graphics card?
The only way I am aware of is by inference from Task Manager -> Performance, where the Total Physical Memory differs from what you know (by other unspecified hardware or software mechanisms!) to be provided by the memory cards in the PC/laptop.
BATcher
Plethora means a lot to me.
It is dependent on the video card specs. For example I have a friend with an Alienware Area51 (2yrs old) with an nVidia 6800 Ultra card and has 3gigs of ram installed. XP sees all 3 and takes nothing for the card that is rated to have 256mb memory. I have a 1yr old XI with an nVidia Quadro 3700 card and 4gigs of ram installed. In my case XP only sees 2.75 
out of a possible 3 (the max in 32bit for XP) as the Quadro card commandeers 256mb along with the 256 it has creating a 512 card. This is not always the case and different video cards – motherboard – memory combination’s may yield different numbers. 
The bottom line is to dig deep into your video cards specs for the specific answer to your personal computers recipe. 
It sure can be hard to track down, both whether, and if so, how much. In my case I built a system with a 128 MB TC card, and knew about this, but still …
The only foolproof way would be to track down the specs at the manufacturer as sinjin says (or remove the card and try to figure out the amount and type of memory and compare to reported size 
).
Then one would know the memory size on the card. How much that is “stolen”, if any, depends on if it is a card with dynamic memory size, and if so it maybe can be reported differently depending on technology used. But at a given RAM size you would only know the maximum it could allocate, not necessarily what is allocated at any given time. An example:
For instance, I have a PC with a GeForce graphics card in the 7 Series (around 2 years old, then in the lower end but still a jump forward at my PC rebuild time). This GeForce-based card has 128 MB onboard memory (at the time they usually had 64 MB or sometimes even 32 MB). It uses a technology found on some GeForce cards that NVIDIA calls TurboCache (and if I remember correct ATI have their HyperMemory technology).
The total graphics RAM can reach 512 MB. At the moment I have 512 MB system RAM, and the graphic card is reported to have 256 MB (128+128), and I think the next threshold is 1 GB RAM, that will make the graphic card reach 512 MB (128+384), or more correct, be able to allocate 512 MB.
Out of the box there is nothing in the OS or graphic driver software that shows the actual amount on the card, as you say. If the card is in a 512 MB system it is reported to have 256 MB period, full stop 
. Then it’s a different matter if it at that moment actually is “borrowing” 128 MB from RAM.
Nvidia Control Panel shows 256 MB
MS System Information shows 256 MB, actually says on card/onboard.
SIW et al says 256 MB.
I would say that even the method with Task Manager you mention is of very little help, at least with NVIDIA’s TC technology. On the PCs I have seen the Total Physical Memory is of the expected size, and since it (TurboCache) dynamically allocates memory I would say it would take a quite experienced person with good knowledge about the actual PC (what it’s running) and how Windows reports Available Memory etc, to observe that there is a discrepancy between Total Physical Memory and Available Physical Memory that shouldn’t be there, if it is possible at all. As to Total Physical Memory my TM reports 523,504 kB.
Example from NVIDIA (a bit dated info. I think it was introduced first for the 6 Series): NVIDIA TurboCache Technology
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