I can manually set the voltages (1.9V DDR2 as memory spec, default CPU voltage, everything else set to auto), CIA2 disabled, 2.0 mem multiplier, all the standard o/c preparation stuff, and the system boots fine as long as the FSB is left at auto or 333Mhz. With everything set the same, I can set the clock control to MANUAL and then change the FSB to 334Mhz, and the system will fail to boot. I've tried other FSB settings 332, 340, 350 - all fail. Set it back to 333Mhz and it boots fine.
Been tearing my hair out trying to figure this one out.