From the nvidia site you mean? Are you using the latest WHQL nvidia driver, latest beta driver from nvidia, or latest unofficial leaked beta driver from another site? Also, do you have any other intensive games on your computer or is this only happening in Crysis and ConvertxtoDvd? Is your system over clocked with custom voltages or is it all on auto?
The freezing could be caused by many things. For example, my rig showed no signs of weakness even with Crysis, but for some reason, the new Source engine would freeze on me after a couple of hours of play. I discovered I had to up the voltage on my ram just a notch, and since then I've never had anymore troubles.