The Durons from 600 to 800 MHz use identical core voltage, leading many to believe that they are essentially the same chip, relabled.  That means that you probably have just as good a chance at hitting 1 GHz with a recently manufactured Duron 600 as with a Duron 800.  Just make sure you are getting a recent week Duron 600.
Also, 1 GHz is a bit high to aim for.  Yes, many chips have hit this speed, but it's not at all a sure thing.
Modus