i always felt that i could push mine until something blew up or started smoking. After seeing screenshots of 2.1 volts on the Vcore & the chip at 6000mhz or shots of 98C core temperatures i thought my 1.4v, 4.2ghz, 74C was nothing really. Just watch out for everyone's accepted limitations on speed, voltage, and heat. Well voltage & heat mostly because you want as much "speed" as possible.
Stay below 75C on a TJmax of 100C, and stay below 1.400v Vcore actual in cpu-z. If you follow those two guidelines, then you can push the speed as high as it will go. Of course, the lower your temps & volts, the better off you CPU will be, and the longer it will live. I'm really interested to see how people fry these things. From what i've read VTT is a major culprit for cpu-death.