X5450 on OEM 775 board RAM incompatibility?

i did the old 771 chip to 775 mod on an OEM HP machine and now it suddenly doesnt like (POST failure) the HP branded Hynix RAM it came with, but works fine with the Samsung stuff someone addded later to it. anyone have any idea what could cause this? id really like to buy some more RAM for this but without knowing what causes this issue i dont want to risk it.