I have seen alot of people who are running 4ghz overclocks on their i7-920's in the 1.25-1.3 range of vcore. I have to kick it up to 1.39 in order to survive prime95. I understand all chips are not created equal but I feel like I am either doing something wrong or my chip just really needs that much voltage.
On the flip side Im not all that worried because after 2 hours on prime95 the highest temp I see is on core #1 at 69 celsius. Playing BF3 and MW3 the highest load I have ever seen on the cpu was 76% and at that point the temp was 63.
Each cpu is different, from different batches. Some websites post their overclocking results so you can see if your batch has a better chance of a high overclock. I don't raise the cpu voltage when I overclock, but if you change the cpu/board often, then you may not care how long the system runs stable with the overclock and voltage increase. Overclocking and raising the voltage stresses the cpu and board components somewhat; can't guarantee if your system will run stable until it croaks. But it's fun.