Right, allocated, but not necessarily used, correct? So, while its nice to have another bucket, it would be even better to know what is actually IN that bucket if anything (depending on the game/res/settings etc).
/we are drifting a bit OT, sorry about that jed!
It's unpredictable. It depends on the game, what is happening in the game and how the game's engine and/or windows decides to manage the resources available. The rational will be in real time.
Realistically there is never enough VRAM so it will be borrowing resources whether from the main memory or HDD or both.
This is an example of the unpredictable nature of memory:
A game duplicates some textures from the VRAM to the main memory, The VRAM is then emptied to store more recent textures. 10 seconds later you change scene in the game and now it has to fetch a portion of those old duplicates from the main memory and copy it back to the VRAM. Maybe the computer realises only 20% of those old textures is needed as you've moved your avater's ingame coordinates to (x,y) the system copies back 80% to the main memory, keeps 10% in the HDD just in case and leaves 10% in the VRAM. You kill somebody and you face coordinates to (x,y) now 2% of the textures on the HDD 1% from the main memory and 4% from the VRAM is needed. The system makes an educated decision to flush the rest of the textures to make room for more. Imagine your computer doing a more complex version of that a million times for every 20 minutes you game.
experience, at 1920x1080, 1.5GB is enough to run this game without hiccups due to vRAM (say 560ti 448c). Yet with 2GB use goes up, and 3GB even more. That tells me we are running out of vram would you agree? Now, here is the kicker.. I play this game at 2560x1440 with a 2GB GTX680 with ZERO hiccups.
Little to do with the VRAM. For example an GTX 680 with 1GB would still outperform a 560ti with 3GB of RAM.
The GTX 680 has a much powerful architecture in general, newer than the 560ti and was designed for more advanced rendering and this is reflected in the price. GTX 680 is high end, 560ti was midrange. The texel rate, pixel rate, memory bandwidth of the GTX680 is in a different league so it had a huge advantage at higher resolutions and with anti aliasing and anisotropic filtering enabled.