I have managed to get all the bits, cpu, ram, gfx, running stable on a Abit IP35 board at 425fsb x8multi.
I cannot even get 400fsb stable on this!

To hit 400fsb with a E6600 should be as easy as shelling pea's!
Maybe it is just me, maybe its just this board does not like 65nm C2D's?
Why dont you take a look at my settings on page 63 and see if you can see what I am doing wrong? But I am following the same procedure that I have used to clock on all boards before.

Lock PCI-e to 100mhz.
Increase fsb in 10mhz increments
When it fails to load windows increase vCore
If this does not help increase MCH volts.