allows the rest of the systam to pass data to your graphics card faster - set it to the maximum you can, it wont make a huge difference but if its there you may aswelll use it, however it can cause instability - if your overclocking then generally they will be a magic figure that will allow it to lock properly, and certain cards prefer certain bus speeds
aint broke dont fix it unless your having problems in my experience however, allough i do know others that have had betetr luck