Assuming your graphical settings are optimal for your video card its GPU usage should be at 99-100% constantly. If settings are below your GPU capabilities you will see a lower number like 40-60 or70%, regardless of which, it should be constant and not varying.
Memory usage will heavily rely on the game engine, your resolution and the level of Antialiasing being used.
My GPU usage is always at 99%!!But what about frame buffer,bus interface,video engine and memory??In frame buffer I have 40-50%,in video engine I have 0-10%,in bus interface I have 30-35% and in memory I have 40%!!Are these ok???