ddr ram has 184 pins. the performance is about 10% better, but it's only worth it to get pc2100 (pc1600 is barely better than pc133). a few mobo's have both regular sdram slots and ddr ram slots (you can only use one kind at a time), but either way it requires a new mobo with either the ali makig1, amd760, or kt266 chipset. if you are building a new amd system, it is worth it to go to ddr.