From my understanding, heat has alot to do with AMD PhenomII overclocking. What cooler do you have and what are your temps?
I only have a 5400+, but some option that I try is to lower HT multiplier and memory multiplier, then raise the base clock slowly from 200 -> 205. that should add about 100mhz. Test the overclock, then continue of it passes.
You might also want to remove 2 sticks of ram, to see if that is hampering your overclock.