Well here is a list of peoples overclocks and what voltages they needed to get it.
As you can tell by looking at the charts. Voltages are all over the place. No two CPU's are equal. Just like snowflakes
They go through revisions, fabrication isn't 100%, etc.
We can guess at what you would need to be stable. And I would say around 1.4. But it is only a guess. The only way for you to know is to try it. You may need more, may need less. May have gone as far as you can. But I doubt you have gone as far as you can.
Also your other components can hinder your overclock.
Make sure you lock the PCI at 100mhz and Sync at 33mhz.