Unless you are playing a game or using graphics acceleration in an program such as a web browser, your GPU usage should be close to 0. Make and model really have nothing to do with it unless your graphics card is completely incapable of any sort of compute acceleration using graphics resources.
Clock speed should vary, based on what you're doing, or as Shneiky mentioned, may be related to power management settings in Windows. When set to High Performance, clocks tend to stay at maximum, but is usually only the case for a CPU. If you have software open performing certain tasks such as video, this can cause the graphics drivers to keep clocks high enough for smooth performance.
Your assumption about clock and memory being fine to run at 100% is correct. The only things to concern about are power consumption if running on a battery and temperature. Those are about the only reasons it would cause a problem, regardless the percentage of use.