if you were to just put pc6000 or soemthing all it would do is just go down to 3200 because it will just underclock it. some people says its faster that way but u can always just raise ur fsb and stuff to handle it.
people put higher rated RAM in the systems because they are going to overclock it. If you put higher rated RAM, then it is guranteed to work at that speed with those timings.
