When you see DDR referenced as 200, 266, 333,400 that is the frequency in Mhz that it is clocked. When it is referenced as pc1600,pc2100, pc2400, 2700 etc that is the theoretical maximum bandwidth in Mb/s,. DDR266=pc2100, DDR333=pc2700, DDR-400=pc3200. RDRAM is rated at the clock speed it runs...