Well you should generally get the number your current ram is because if you currently have PC-3200 Ram and you buy a stick of PC-2100 Ram then all your ram will operate at PC-2100 Ram speed. The higher the number the number by the PC- Generally the better type of ram as long as the sticks of ram you are comparing are of the same kind. You wouldn't want to compare a stick of 512mb PC 2100 Ram with a stick of 512mb DDR2 Ram @ 533MHz because they are in a different class, just the same you wouldn't want to compare DDR2 ram to RDRAM. I hope I made this understandable. Anymore questions just ask and I will try to explain it better, If I can't somebody else here can.