In the case for gaming how much will a RAM's Frequency and size matter? Say for 4k gaming will there be a significant difference from say my current set of 4x8GB 2666 MHZ with a 2x16GB 3600 MHZ?
check out the Youtube review below about to see how different Ram performence with the ryzen 5000.
AMD Ryzen: 4 vs. 2 Sticks of RAM on R5 5600X for Up to 10% Better Performance
It might helping u for your desicion.
for Ryzen 5000, the benefit of fast RAM has gotten smaller. In general, RAM speed (frequency and latency) is more important in CPU limited applications. You're aimed 4K gaming should be clearly VGA limited, not CPU limited.
Like Zen2 you can say about Zen3, that DDR4-3600 is a sweet-spot. Of course you could aim for DDR4-4000 with 1:1 IF ratio, but the benefit is small. Latency optimized DDR4-3600 should be the best bet without the hassle to tune 4000 MT/s FCLK.
Last, Dual Rank memory has some advantages compared to Single Rank config, because the memory controller can utilize "rank interleaving".
I would recommend for a new Ryzen system (Zen2 oder Zen3) DDR4-3600 with at least CL16-16-16-36 (lower is better). And I would recommend 4x8GB Single Rank modules or 2x16GB Dual Rank modules. I personally ordered the new G.Skill 2x16GB DDR4-3600 CL14-15-15-35 kit ;-)