Hi I figured I should inform you of some painfully obvious facts.
As you already know and have been informed of dual-channel is faster than single and for that reason a single 8GB module isn't viable in comparison to 2x4GB where these two options are the only available ones to choose from. Also when upgrading you generally can't simply add a new RAM module and you would be fine.
Upgrading normally means losing the previous kit and changing it to a new kit so with the single 8GB module it is not adding three more modules, but buying a set of 4x8GB modules to have a 32GB RAM kit.
For a RAM kit to come together it normally needs to be of the same revision and manufacturer for maximum compatibility without intermittent issues (which are sometimes seen in mismatched RAM kits), but they must be of the same specifications otherwise there will be issues that will occur whether it is ones which prevent machines from powering up or ones which cause unusual issues such as BSOD after booting up either immediately or after a while.
I recently tried buying the "Red Venom" equivalent of my current RAM, but because they were sold as singles this was fair disastrous in that all four modules were very different to each other in terms of revisions and batch numbers being far a part from each other. One had black PCB, three had Green PCB, but the rest of the features were the same, and as mentioned before the batch numbers were far apart from each other which caused compatibility issues. My bad luck was that one module (green PCB module) was faulty (prevented PC from powering up at all) so I could only use half the kit and one of the Green PCB modules would only work with the other Green PCB module and not the Black PCB module.
I ended up being refunded for the kit, went to another website and brought my current RAM which was sold as a matched kit. To date I have not had any issues at all and none relating to my RAM.
If you are upgrading buy the full upgrade immediately and not buy half the upgrade now and the other half later as it is likely to not work so well as is the same as buying two of the same RAM kit to make one, although sometimes you can be lucky; however I still won't recommend buying singles or two dual channel kits to make a four or more RAM module kit.