The answer is yes, you can mix EDO memory of 60 and 70ns times, but as Andy pointed out, the memory will run at the 'lowest common denominator' speed -- 70ns. In most cases that's not a problem, but some of the older socket 7 boards don't like it, and you'll get weird errors from time to time. You can't harm anything in trying it, so give it a whirl and see what develops.