Is there a way to tell if the northbridge is holding you back when overclocking? I am considering replacing the thermal paste on the northbridge heatsink with some Arctic Silver 5 and see if temps drop. I'll lose my warranty, but the board was only $90.
The NB was some white paste and was all dried up.
I also installed a 40mm low rpm fan on the NB sink. It all helped in getting my E8400 to 4GHZ.. Before the HS mods all I could get stable was 3.8GHZ.