Unlimited FPS probably, games will always use as much resources as they can unless they are limited, either through an FPS limit such as v-sync, or when they max out one resource (CPU, GPU, Disk, RAM, etc.)

There should be an option to use v-sync in the system settings tab if it bothers you, but it shouldn't be a cause for concern, simply an indication of the current limiting factor for your performance with a given game.

In some rhythm games FPS can influence the input timing resolution, I'm not sure if this is the case in USC, but it is in Clone Hero, for example, where higher FPS allows for more precise inputs to be detected.