Well it all depends on your mobo if it has dual channel, what is the bus speed. Ideally, you should put RAM which have the same frequency as your bus. If your bus is 400 Mhz, I would not only put PC3200 (Mhz) but I would change your 333 Mhz stick (which is a PC2700 BTW) for another PC3200 stick and set in in dual channel mode if your mobo is set for it.