Well here is a list of peoples overclocks and what voltages they needed to get it.
http://www.hardforum.com/showthread....2&page=1&pp=20
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.