Elio I had the same Issue with my other Q6600 L723A couldn't reach more than 460x7 and couldn't boot at 450x9

I have a P5K deluxe and i love this mobo, i started to think that it was her causing that wall but i was wrong it was the quad...I now have a Q6600 L737B and can do 485x7, 450x9 and all mobo settings at min values (SB,NB,PLL)...So the wall was the batch...

Cheers
Sergio