usually pc133 is backward compatable with pc100, so i am assuming you are using pc100 64mg of ram and want to add pc133 128mg stick as mentioned in above reply, so you would end up with 192 mgs of ram with both sticks, but like said above, it would run at 100 mhz due to the pc 100 64mg stick, but you would hardly notice the diference anyway in my opinion. But it should work fine for ya.