Ok, I'll explain it more simply, as I don't feel you are quite grasping the usage concept, which isn't exactly an easy concept to grasp.
You go to hang a picture. That's going to require you put a nail into the wall. When you grab the hammer, it's going to mean that you will use 100% of the muscles in your hand, every tendon, every ligament is used. That's what the gpu does, it uses everything it has, all 100%.
Now you go to swing the hammer, you'll again use every muscle in your wrist and arm.
Usage isn't what's used, it's what uses. So gripping that hammer, swinging that hammer, you'll use every muscle, but you will Not use all the strength you have in either hand nor arm. You do not require a death grip on the hammer, nor require swinging that hammer with all your might. The job does not need or require that you bring to bear All your strength.
That's usage. The amount of resources (strength) you need to use to get the job done. Increasing difficulty, like if you find a stud, may require you to swing harder, use more strength, to get the nail sunk enough (higher complexity frames) . If you were at 100% usage, you could not swing the hammer harder, so the nail needs to be hit more often, that makes the job longer per nail, so pictures hung per hour (fps) goes down.
1% usage or 99% usage or anything in between is the same amount, all that's necessary to get the job done. The Only number that differs is 100% usage, because that's a guarantee that if just 1 small factor, one tiny particle, adds to complexity of the frame, you have Zero strength headroom, fps will drop. You are Far better off with a gpu at 70% usage than a gpu at 99% usage, you have far more headroom for complexity changes, like explosions, going into heavier npc Ai populated areas, fields etc.
Switching to dual channel ram means the ram is shoveling @ 1.5x more data per cycle than single channel. That means the cpu no longer can sit on its butt and take its sweet time per frame, it's now in a hurry to process all that extra data. Usage goes up. Also means it's sending more info per packet to the gpu. Gpu usage goes up. Fps goes up because detail levels are not having as great an affect on the fps packets sent by the cpu. That can mean smoother frame transitions, less stutter, less impact on v-sync...
The way ram channels work is parachuters jumping out of an airplane. Single channel is the guys in a straight line jumping, waiting for the guy in front to clear, next guy jumps. In a straight line, so there has to be a wait period for the guy to clear the tail so you don't hit him.
With dual channel, it's 2x lines and you don't jump straight out, but on an angle, an X, / then \. So right after one guy jumps to the left, another from the other line can jump to the right. So while you still have the wait to clear period, that's only affecting 1 line at a time, the other line is already clear to jump, filling in the gap.
Single channel : °/°/°/°/°
Dual channel : %%%%%