I take it being on an asrock board that its newer than the cpu
mulitplier will be locked on the cpu so your totally dependant on fsb.also i would imagine its a 100 fsb or 133?? on a clock mulitiplier of 9 or 6? Did you have to underclock the board?? What ram are you using as this could have an effect.
If it is on a newer board and your using ddr the only restraints I can see will be either temp of cpu or cpu threshold reached. Up the fsb in 5's at first if instability is reached before to high a temp find vcore and up this a point or 2
I may be off track on the asrock but I cant remember them past a couple of years ago