MB is GA-B75M-HD3, it supports dual and XMP. I just was not sure if buying an unmatched stick, even though it has the same product number was 100% going to work dual channel.
It definitely won't if the new stick is of a different size. It MIGHT if the voltage and latency specs overlap at the desired speed.
Point of it -- you'll get "more capacity" with the unmatched sticks, but lose the dual-channel operation.
I went from 8GB to 16GB kits about a year ago. I didn't really need it, nice as it is. NOW it's only especially nice, because it most definitely supports increased "RAPID" feature of my Samsung SSD and second version of the Sammy Magician software. But a single 8GB kit MAY have worked just as well for that feature -- a feature that most of us SSD hot-dawgs never really contemplated, even as we purchased our first Sammies.
If you're happy with single-channel mode, you could conceivably buy the 8GB module now and replace the 4GB module later with the same model. But really -- from two different perspectives -- it is not a crucial cost-benefit issue. [no brand-name-pun intended.]
First, the price on single memory modules might be comparable to five Mexican drive-thru dinners.
Second, by the time you really think you "need" 16GB, it still won't be a matter of "need" unless you're doing some very specialized things with your system -- in which 8GB might still fill the bill, anyway.
But you're looking for opinions. My own opinion: get an identical module to the one you have, or simply replace it with a 2x8 kit. But the 2x8 kit is more than a lot of folks will need -- which means more than they will effectively use.