Personnaly I just leave it to auto, I'm not absolutley certain what it does. But as the old saying goes "if it aint broke, don't fix it" (however if it is broke and no longer under warranty, hit it with a B.F.H.)
(Big F***ing Hammer)
Does the system have onboard vid? that could be why vista shows - total memory available to graphics 756mb.
Vista visualizes memory for the video card to swap out textures when the VRAM is full, sort of like a page file. Nothing to worry about, it isn't taking away from your ram or any system performance.
The Robust Grahpics booster is a bios controlled video card OC. If you aren't very good with OCing and just want a nice little boost in performance than turn it on turbo and see if you like the results.
I actually played around and did an experiment with that. It seems to provide a very small improvement. This was just using 3dmark06 however, without changing any other thing with the exception of the RGB from fast to turbo. I dunno just put it on turbo, its sounds cooler than what it gives out. Then you can say your graphics card has a turbo.