I do not know the answer to this Chivas but I would say:
#1 the bottleneck is the VRAM on the GPU. As soon as it gets full, problems start. I have seen COD producing "slight terrain stutter" on PCs with small GPUs (512Mb VRAM) much faster than on my PC (1,3Gb VRAM),
#2 my assumption using plain logic is that data will move from the HDD to the CPU first and then to the GPU as the CPU will decide which data needs to be loaded (This is an assumption!).
This may be of interest:
http://www.tweakguides.com/Graphics_3.html
~S~