Overclocking E8400, confused about BIOS setting on 790i mobo

Mar 6, 2006
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I'm trying to overclock my intel Core 2 E8400 3GHz CPU. So far I have it at 3.8GHz with a slight increase in voltage to the core (1.4v up from 1.3v), on air cooling.

I want to hit 4.0GHz but I'm confused about a setting in the BIOS. It's an nForce 790i motherboard and under voltage settings there's an option to change the CPU FSB voltage as well as CPU core voltage. What is this CPU FSB voltage setting? Should it be increased with the core voltage? What should I set it to (it's at 1.3v at the moment)?