It first of all depends on witch revision it has the old CO or the newer DO. DO is the bedst one to overclock since this later revision needs les voltage and by that produce less heat.
You can see on top of the CPU what revision it is or if its up an running use CPU-Z to confirm it. If its a DO they are normal good for around 4 GHz or more.
What i can tell also is that my i7 920 runs hotter than that i7 980x i have now.
Se if this riden on the headspreaders top.
C0 (SLBCH)
D0 (Q1H7, SLBEJ)
Else are here a comparison for my old I7 920 at 4.3 GHz and my I7 980X.
I7 920
I7 980X