on the P5Q board you can alter the DRAM:FSB Ratio, so you can push the FSB higher, so it doesn't play about with your RAM @ 1 : 1
You should be able to select 800Mhz on the DRAM option and it will auto set the ration for you. Mine runs at 4:5.
I also see that it's C0 stepping, but I'm sure that people with the same chips have got 4GHz
Try setting your FSB to 421 and the multi to 9.5x
Then make sure the DRAM is set to 800MHz. You could get away with a 1:1 and run your RAM @ 842Mhz, as it's only a slight overclock, but it's up2 you.
Up the Volts on the CPU core to 1.4 and see how you go.
Then run Prime and watch the temps. If they are fine, then lower the voltage down a bit and see if you get the same crash.
As you lower the voltage you should find the optimum voltage with a good stability : Heat Ratio.
HOWEVER - - - Someone else may know better, so if I've posted anything thats BS, please correct me.