2GB is possible I know this because my HP Pavilion a305w is running it right this minute. DDR PC2700 133MHz is what spec. calls for within the manual. But, PC3200 166MHz does work as well. Higher MHz gives more room for computation and ups the speed of computations. In other words, faster input to the memory and output to the processor. PC2100 also works, but is not recommended. Between PC2700 133MHz and PC3200 166MHz (Both work exceptionally well for this brand of computer.