all chips like ram, processors come with a specific frequency.. for eg. ur ram might be a 667 mhz one or a 800 mhz one, the latter being better.. so if ur existing RAM is of 667 mhz, then u have to put a RAM of 667 mhz only... unless u want to celebrate diwali a bit early... also take care that the difference between ur 2 RAM chips is not much... a 512mb and a 1gb ram wud go fine, but a 256 mb ram with 1 gb might create heating problms... nevr try o put a 128 mb wid a 1gb RAM guys!!! i did it once, n enjoyed a little boom boom boom....