This is normal for mobile GPUs. At one point nVidia made a system where card would be able to use both its own RAM and system RAM. After that they presented some graphics cards with 32 MiB and 64 MiB of RAM. They could use system RAM, so their total amount of RAM would be 128 MiB and 256 MiB, if I remember correctly. Basically, the dedicated video memory is the card's RAM and the shared memory is the main RAM that the card can use when it needs extra memory.
That system remained in their mobile graphics cards. As far as I know, there is no way to check if the card is using that memory or not. I myself have a GeForce 9500M GS on a 4 GiB system and have never had problems with card eating any of my system RAM. On my computer, the shared RAM is 1790 MiB, so it could be that driver automatically sets a certain percentage of system RAM as shared.
EDIT: After doing some calculations, it seems that the shared memory is set to around 43.7% of main system RAM.
Here is article about that technology on NVIDIA's site. Here's another interesting article on the technology.
From what I've read,
you won't have any problems with TurboCache using up needed RAM because it only uses RAM not used by other applications and it only uses RAM when it needs to to so and when it will improve performance.