Overclocked My e6400 to 3.35GHz (419fsb, which is reported max for this chipset) and is Prime95, 3DMARK06, and Crysis stable.
I have the Gigabyte EP43-DS3L, with 667MHz MHz at 5-5-5-12 (got it to around 838MHz 4-4-4-12 @ 2.2v)
No bump in voltage, no tweak in settings, nothing will work...