How far could i pretty safely push the Core 2 Duo E4400 (C) DC 2.0 GHz

most i see stop at 3.4ghz on p35. rarely someone can go above it in reasonable vcore.

I got 2.8GHz with small voltage increase(1.34 after v-droop) and it was cool on my Zalman CNPS 7000. With a bit more voltage I was able to get it up to 3.0GHz. It ran without going above the rated voltage at up to 2.66GHz. This was all done on my Asus P5B-Plus(Intel P965 northbridge/ICH8R Southbridge) I did not adjust chipset voltages or FSB termination voltage for 2.8GHz, I bumped NB and FSB voltage one notch for 3.0GHz.

I got a E4600 and it's max OC is 3.3 which is dumb. I should have gotten an E4400 or E4500 which OC to 3.4-3.5.

actually they all OC to 3.2-3.4ghz.if you want more you will have to upgrade to E6xxx series. as the lower E4xxx and E2xxx is really bad yield speed binned parts.

I got mine to 3.15 with no volt increase on CPU, ram or chipset.

Aye but i am in a stock HP box so i cant overclock through mobo, what programs do you suggest i use to overclock?

use to oc a hp?pin mod is the only way.

Other than hard mod, I'm not sure if there's anyway to OC your system. Using Ntune doesn't work either.