Don't know of the highest, but there was an article about a couple of 1.26 tullys oc'ed to 1.5 (166 FSB) each with just a heatsink....no fan!
I would assume that if decent fans were introduced, the potential is very high. Doesn't the core voltage maximum prevent going too far beyond this?
<b><font color=blue>~ What do you mean "It isn't working!"...Now where's my sonic screwdriver? ~ </font color=blue></b>