You can lower it, untill the pc starts lagging. It can't harm the pc. Those speeds you mentioned are the stock settings btw for a 5670, i even think the memory is 775 standard so it's underclocked allready ?
More important ; when your not using it's full potential ( when doing normal work with it ) it doesn't run on those high speeds, it will be about the half of that.
My runs at 950Mhz while gaming and internet use and around 450 i believe while working with Word.
thanks for answering, the only time I'm putting it back to 775 is when I'll be gaming I guess, btw, would it be best to have fan on automatic or should I manually put it to a set speed?