Here's what I've found so far...

I needed to up vCore to 1.4250v in BIOS (1.48v actual in PC Probe) to get PC to POST properly, regardless of multiplier. Overvolting problem is still there. It seems even though I'm underclocking my CPU at 6x405 (2430Mhz), the motherboard behaves - at least at POST - as if its still on default (for me 10x).

Once in Windows I was able to Clockgen the FSB up to 450Mhz. Tried 475Mhz but it locked up. I guess I would need more vCore.

I'm not prepared to stick 1.6v through my CPU at air cooling so I'll resume testing once I've got my Vapo. I suspect the board will hit 500FSB with ease, though if my suspicions are correct I might not be able to boot it at 500FSB because it would be trying to boot at 10x500 (5Ghz) on an E6700.

My board is a retail board bought from a shop, which is totally stock at the moment (haven't even removed the wrapping on the NB cover or changed the TIM)

I could be wrong though....