Honestly, if you have asked around Tech Repository, Tony should be able to aide you through this better than me.
However, I can say that it looks like your CPU is maxed out. The e4300s that I have seen have not been OCing as well as the old e6300s, and not even the new lower-end model Conroes (e6300 & 6400) have been clocking as high. My guess is Kentsfield binning.
Also, I dont think you need to adjust chipset voltage. P5B easily goes above 450; mid 300s should be incredibily easy. vdimm should, also, only help when you reach higher bus speeds. (the 2.1 limit should not hinder your achieving of 400+ fsb)
freecableguy and Tony are the ones to ask though