I think monotone meant to say, 1.6v is nominal for a duron. The specs are 1.5-1.7v. But all that aside, you might of unfortunately bought your duron from someone who had already gone through the bin and found those that overclock well and those that don't. AMD only guarantee's 600 from a 600, but there is big difference in actual chips. Most 600's would do at least 800 though. Have you tried a different chip in that mobo and parts combo? See if something else ain't limiting you? If not your chip is bad. But on the bright side it runs a little faster.