The more the merrier! More ram almost always helps especially for editting photos but this only applies if you make full use of all your ram. The huge jump from 64 to 128gb probably wouldn't make any difference at all for the average user.
It is generally better to have 2 identical sticks of the same RAM as the computer can then access them individually and thus get things done faster this is called "Dual-channel". It's not essential and the performance increase is not as much as the description suggests. You'll be fine.
Also, I don't think dual channel technology is specific to dual-core systems. It should provide some benefit to all. I'm not exactly sure though.