When I first upgraded to my Q9650 from an E6300, I didn't clear the CMOS. It booted fine but the BIOS got confused. It wanted to run at the old 266 FSB instead of 333 and it wanted to run the voltage at 1.35V instead of 1.25V.
I ended up manually setting up everything in the BIOS, but I have since cleared the CMOS and everything was detected properly with all CPU settings in the BIOS set to "auto".
I suggest you clear the CMOS before the upgrade just in case your P5B gets confused like mine did. It's always best to start with a clean slate IMO.