The other thing i noticed that my old RAM is dual channel (i think, because it has the "black chip thing" on both sides of the memory stick) and the new RAM only has it on one side.
that is double sided, not dual channel. however you may be right, there have been compatibility issues in the past when it comes to mixing double and single sided memory.
though if you have indeed tried the new memory on it's own in the slot the old memory was in, i am inclined to suspect the new memory is faulty. it should work fine on it's own.