I have to say that you have heard wrong. GDDRN has nothing to do with DDRN, one is solely for the use of the GPU, the other is for the CPU. There is no interaction at a specification level. This question comes up time and time again, you can use any memory on the GPU with any memory on the CPU.
The 730 is barely better than modern iGPU's, try to get to a 750/750ti if possible.