<font color=blue>depends if your mobo has a 133/266 fsb or 100/200. 1600 is pc100 doubled to 200, PC2100 is pc133 doubled to 266. Opt for the 2100 if your comp can handle it. A cs of 2 is great obviously, but I would go with 2100 & 2.5 - if compatible with your system.