Modern GPUs will run into a hybrid mode where the drivers/GPU start streaming texture data from system RAM over the PCIe bus to make up for the "missing" RAM. Since system RAM is 3-5X slower than GDDR5 with much higher latency, running out of "VRAM" would translate into a significant FPS loss.
In most games though, you are not likely to run out of GPU RAM unless you run with a high level of FSAA or other memory-hungry feature.
Oh.. So running out of VRAM would cause a huge FPS drop.
So you're saying that what I'm experiencing in BF3 (a drop to 50 fps) is not because I'm running out of VRAM?
How low do you think the FPS would drop if I were to run out of VRAM?