You can always pick up another 2x8gb set down the road, maybe even try looking on ebay when the 8-10% coupons come around. Unfortunately there is really no good way to run 32gb+ of ram without potentially sacrificing OC headroom or by going for a motherboard specifically designed with this in mind like the X299 Dark, but that is an entirely different platform. 4x8gb sticks are usually the best when it comes to compatibility, but the key things to make sure when adding 2 additional sticks down the line if you intend to keep what you have purchased is to make sure it is the same exact model number (no difference like a z vs an r at the end of the model number) as well as making sure that the timings and sub timings are the same. If they are a decent OC can be as quick as an XMP profile, and if not you may run into limited OC potential or worst case instability/incompatibility.
As a rule of thumb when selecting DDR4, you really want to make sure that the CAS is less than half of the first two digits of the speed. I.E. DDR4 3200 cas 14, 14 is less than half of 32. DDR4 3600 cas 16, 16 is less than half of 36. This is a very general rule of thumb, but if you stick with it you are almost guaranteed to get a decent performing set if not b-die which should have the added benefit of additional couple hundred mhz OC room and/or tighter timings with good tuning.
Gigabyte Z370 Gaming 7
EVGA RTX 2080 Ti XC ULTRA
Kingston HyperX Predator RGB
ADATA XPG SX8200
EVGA SUPERNOVA G2
EK Velocity Nickel-Plexi, EK Vector RTX Nickel-Plexi, EK EX360, Koolance 360mm, Enermax Neochanger
Logitech G502 Spectrum
Audiotechnica ATH-900X, SBX G5, MODMIC
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