The 6600 is a pretty low end video card. It is reasonable to assume you would have to turn down the graphics a bit to get smooth gameplay. I have a 7600GT and cannot run with the "Detailed Terrain" option enabled or my frame rate drops to low to keep the game playing smooth.
The video RAM is not something we have any control over. NVidia's latest drivers use system RAM, when they can, instead of video RAM.