I believe Intel has advised 1.365v as the recommended maximum vCore. While I have tested at 1.6v (4.6GHz) on air for 30 minutes or so, I don't see any degradation in mine, still o/c's at the same voltages as previously. I could eke out a bit higher FSB with this voltage, but I am satisfied with 4.0GHz and it is not flaky in any way. Here is my bios settings for a rock-solid o/c:

Ratio CMOS Setting : 9
FSB Frequency : 445
FSB Strap to North Bridge : 266
PCI-E Frequency: 100
DRAM Frequency: DDR2-1112
DRAM Command Rate : 2T
DRAM Timing Control: Manual
5-5-5-15, trfc 25

DRAM Static Read Control: ENABLED
Transaction Booster : DISABLED
Boost:0
PERFORMANCE LEVEL: 6

CPU Voltage : 1.365v
CPU PLL Voltage : 1.4v (Lowest Bios Setting)
North Bridge Voltage : 1.25v (Lowest Bios Setting)
DRAM Voltage : 2.1v
FSB Termination Voltage : 1.20v (Lowest Bios Setting)
South Bridge Voltage : Auto
Loadline Calibration : Enabled
CPU GTL Reference : Auto
North Bridge GTL Reference : Auto
Clock Voltage Over-Charging: Auto

CPU Spread Spectrum : Disabled
PCIE Spread Spectrum : Disabled