Just thought I'd add my experiences on the subject. I'm running a Q9650 on my P5B-Deluxe-WiFi/AP with the 1238 BIOS without any issues (upgraded from the 1st gen E6300).
The install went fine and on first boot, as I recall, a POST popup appeared complaining about the CPU. I went into BIOS and had to manually set the FSB to 333MHz. While in there, I locked the PCIe to 100MHz and the PCI to 33MHz since technically running at 333 is 'overclocking'. Everything else CPU related was left at default or auto. No more POST notifications after that. On a side note, I'm running my RAM 1:1 at DDR2-667 speed even though it's DDR2-800 memory. The memory divider in the BIOS sets the RAM at DDR2-833 when changed to the setting that previously with a 266 FSB resulted in DDR2-667 memory speed, and the divider that previously was used for DDR2-800 now results in DDR2-1000.
After booting into Windows, I ran CPU-Z and noticed that the voltage was a little high for a Penryn CPU... about 1.32V or so. Using Core Temp, I found the VID for my Q9650 to be 1.25V so I went into the BIOS and set the CPU voltage from 'auto' to 1.2500V. CPU-Z now shows the voltage to be about 1.21V after VDroop.
Anyway... I've been perfectly happy with my Q9650 in this motherboard and it has been 100% stable at stock. I haven't felt the need to OC so I can't comment on that. The only thing that doesn't quite work I believe is that Core Temp shows the VID to be 1.10V when speedstep kicks in and I've never seen my CPU voltage drop that low when using speedstep, but that's a very small issue.
Hope this helps.
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