What is important when choosing a RAM module are this factors:

Type of RAM: in your case DDR.

The speed of memory: 266MHz .This speed is related to the Front Side Bus (FSB) speed of your processor. In short, FSB is the speed at which all components inside your computer communicate with one another.

DDR memory operating at 333MHz actually operates at 166MHz * 2 ( PC333 / PC2700) or 133MHz*2 (PC266 / PC2100).

You should choose for your computer a DDR PC2100 module. More RAM in your computer is better.