DDR is 184pins & PC100 or PC133 DIMM's are 168 pins. DDR is DOUBLE DATA RATE dimms hence the name DDR they operate at a faster access time (or seek time is another name for it) basically it is the time from when the cpu first request data till it recieves it. DDR is better than pc100 or pc133 but only if you have the Motherboard, & Cpu to handle it, for ex. AMD's XP line was made to be DDR compatible. As MasterC said get a mobo with both slots (the DDR slots will be a little longer than the pc100 or pc133) use your old ram with your new mobo & cpu. Another idea is that some wedsites offer a bundle (Mobo, Cpu, Ram, powersupplies or some combonation of these) they may be a better deal there do some shopping before you buy.