Originally Posted by name='k4p84'
They are different frequencies, that will not help. ie one is 800mhz, the other is 667mhz.
Spot on, it will depend a whole lot on how flexible the mobo allows the cpu/ram to be.
Some laptops u can get away with it, quality of the ram usually making a difference if ur going higher with the clock. POS ram usually allows an underclock from it's stock - as long as it's stated as being able to. Even expensive quality memory won't do it unless stated (or unless lucky).
Usually best, if ur not up with messing around with them, to go with the rating the existing memory is, or higher. If it works lower - ur fortunate, not a gamble I'd spend cash on tho.