How should I go about upgrading my video card on a macbook?

Basically i recently installed the sims 3 and like many other sims 3 players, i have experienced regular game freezes. I had heard by not having the most recent version of the video card on your computer you may be contributing to the problem. I worked out that I currently have NVIDIA GeForce 320M video card am I am not sure if this should be upgraded? I am pretty much technology illiterate, so I am hoping someone can tell me whether there is a download on the nvidia website that might improve gameplay on the sims 3 on my mac!