1,6ghz is way too less. i have 3ghz 3gb gf9600gt and also have 40 to 60 on maps with more people, with less fps go up to 100 but are never stable at that.
the reason why lowunstable fps suck is that you have input lag because of it (at least i can feel it on my comp).
your actions are not smootly translated into the game as well as stuff being shown to you is choppy.