What he meant is probably based on the fact that if you are using double channel memory access mode then cpu can read from other module while first one is busy and since he said it was 6 GB so probably (4GB + 2GB) which means when you fill memory to 4 GB mark it will not have free module to alternate and performance will degrade. But this is all in theory and highly speculative in practice you would probably see 10-15% ram speed degradation not a 50% as you would expect. So unless there were some other issues like mismatched ram frequency the statement that is has to be in the powers of 2 is not really a hard rule, to perform optimally you want it to be but it is not strictly necessary.