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I have the same board, different C2Q CPU (Q9550) but having the same problem when trying to go beyond 399 MHzx8.5 in BIOS. I can go 450 MHz+ and the machine will boot Windows just fine and stable, however I get the Waiting for Root Device and "no entry" symbol when trying to boot off of my OS X HD at anything higher than 399 MHz. So, I installed a $12 Silicon Image 3132-based x1 PCIe SATA controller and OS X drivers and voila! Machine boots perfectly at 450 MHzx8.5 into OS X. Onboard PATA controller works perfectly at overclocked speed, BTW. I would guess that it's some kind of OS X-specific ATA driver issue since SATA works perfectly with an add-on card (seen as SCSI) but not with onboard when clocked above 399 MHz. Funny that PATA works perfectly though (my Pioneer DVD-RW). Would love someone to solve this.
I was curious whether anyone has the 3870x2 working under any flavor of 10.5.6... I've followed about every guide out there. EFIStudio strings, hex editing, pkg installers, etc. You name it, I've tried it. Since I dual-boot with Vista, I'd like to have my 3870x2 working under both configurations... Thx, Shane