If you provided slightly more information, that would be helpful. RAM timings (at stock speed, as well as at OC), RAM voltage, chipset voltage, HTT multiplier, CPU voltage, and CPU multiplier ought to get us started. Just a shot in the dark here, but try your 300Mhz overclock again, but manually set your HTT multiplier to 4x. See if that boots and post back.
Also, gather this other information and post that as well, and we'll see how far that bad boy will go.
-J