So far what's I've found is that 1.2VHT & CPU VTT play big role getting stable overclocks.. For my P5N32-E SLI (0903) & E6600 those sweet values are:

1.2VHT = 1.20 (BIOS)(1.26 in Windows)
CPU VTT = 1.30 (BIOS)(1.36 in Windows)
NB = 1.40 (BIOS)(1.44 in Windows)

With these settings I've archieved 12 hours with 400x9 with quite low voltage levels.



I've also tried to go further over 400 fsb, with Auto setting board sets

1.2VHT to 1.46
CPU VTT to 1.46

when this happens overclocks are unstable, when I manually set them to 1.20 and 1.30 board is stable again. Have you noticed same kind of behavior?