Yeah I'd say that if you are looking into a GPU upgrade, I'd put that money towards it rather than a RAM upgrade.
The other thing is that buying DIMM's piece meal is a gamble, even if you think you are buying the exact same part number as what you have. Sometimes when a memory manufacturer starts a run of a certain part number, later on they may not be able to get the exact same memory, so they swap them with something "comparable", but they don't change the part number. Usually the RAM IC's are very similar, but there are always slight changes in the secondary and tertiary timings. These "smal" differences can cause issues when mixed with RAM with slightly different timings. It could cause the motherboard to not POST, or cause weird random (hard to troubleshoot) crashes, freezes, etc.
So my advise to you is, if you go with 16GB, get a 16GB dual channel kit and sell or re-use the RAM you have somewhere else.