I was running my E3110 (same as E8400) at 500*8 @ 1.3375v. It is recommended to keep the vcore below 1.4v
Grab real temp to check your temperatures, make sure the TjMax is set to 95°C
EDIT: Have you increased your northbridge voltage at all??
Are you running you memory unlinked or are you ocing the memory as well?
EDIT2: Data base from my other forum (note that the vcore shown in the table below is read in Windows under 100% load)