there is no set speed to which you can over-clock a certain cpu . there are too many variables , not the least of which is being lucky enough to get a cpu that clocks up well . the 566 appears to be your best bet though as speeds of 850+ are very common , ive seen the 600 up there as well , but i think youd have a better chance with the 566. just make sure your case air flow is good , and you have a good HSF , i use a Global-Win FOP-32 on mine.