Hi, and welcome to the boards mate..
With IOH core voltage I just set that up a few increments from stock, as for ICH i/o and core, I just leave those on Auto, as I don't think they have any effect on overclocking, as this is just the southbridge voltages.. though I could be completely wrong here, maybe someone else can chime in here and explain this in more detail?
Don't some C0 stepping i920's require the QPI/Vtt to be up to like 1.45v, and around 1.400v for Vcore to get 4GHz?
Have a look at the Gigabyte guide if you haven't done so already...
http://lakesidepc.com/Core.i7.920.oc....12.12_ENG.pdf
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