as long as you have pc133 memory then 133 is the best option as it runs the memory at 133mhz which gives more bandwidth, if you only have pc100 memory it may still run at 133mhz as long as it's good quality, for example I have 2 pc100mhz dimms, one is cl2 one is cl3, the cl3 one will run at 133mhz at cl3 and the cl2 will run at 133mhz cl2