I got 2x 512 mb samsungs that cpu-z states are pc2 4300 (533) ddr2. Yet when i put my serial # in my manuf. site it states the config. is 4200 (533). Who is right? Secondly, i know my pc can take 4200, 5300 & 6400 rated memory. Will adding 2x 512mb sticks of either (either 2 sticks of 5300 or 2 of 6400) to my remaining 2 empty slots slow or hurt my pc. I would be having 2 different groups of memory rated at different speeds. Or is it much better to get similar speed sticks.