The last cpu I used in my ax6bc was a celeron 900 running at 1200mz. The 133 fsb made the agp overspec but no grief from the asus 6800 I was using. It was very fast and I regret selling it. I now have a couple asus motherboards with tualitan celerons and can overclock the 1.1 to 1.466 but its not stable/consistent so no better performance for me over the 1200.
You would have to get a slotkit adaptor, power leap adaptor and 1.0 tualitan that should make 1333 but would be very expensive. There is also a wire mod that can be done to the tualitan cpu, shorting out some pins that I read about but haven't done.
Good luck & I sure miss my old ax6bc.