Adding more ram can help when it comes to loading and unloading memory. The only time you will you see stuttering is when it must unload data such as scenery or objects. But if you have enough RAM, the better the chances of the game LOADing the data, then unload unused data in the background. This is why people with more RAM wont notice the stuttering.
Its also useful if you run a lot of applications in the background. If you have enough RAM, you can play the game, run skype, browsers, and tons of other applications with no problem. I have 12gigs and will soon get 24. Im a 3D Artist and need the ram!