I don't have a 2600k... Yet haha.
It will be hitting 4.5ghz as that seems to be the common result for most chips on the latest stepping. So I realistically cannot see mine being much lower as long as I get all the supporting bits.
My E8400 however sits @ 4.1ghz 24x7, and has since Nov 2008
Was @ 4.3ghz but when the ambient is about 27-28c it gets angry at me. Not sure about mobo temps, either way they aren't high as I've not really changed them. Only alteration is the vcore which is 1.32 @ idle and 1.28 @ load so a bit of droop, but nothing overtly worrying for what was a mid-range board back then.