Simply put, you're not going to find that any game is RAM bound with a discrete GPU. Core i3 vs i5, doesn't matter. You'd be better off with the cheapest RAM and the i5 than the fastest RAM and the i3.
While I understand that the names DDR/DDR2/DDR3/DDR4 are not arbitrary and are backed by JEDEC standards, it is fundamentally true that DDR3 2400MHz at CAS11 would perform identically to DDR4 2400MHz at CAS11. If the clock and timings are the same, the technology doesn't matter (sure, single/dual/quad channel would matter, but assume those are the same).
It's completely unsurprising that the only area where RAM makes any difference is in cryptography, encoding/decoding, and physics. For games... you're just wasting time thinking about it. DDR3-1600 @ CAS9 or better is all you need. You can literally double the memory bandwidth and you see no improvement at ultra settings, only at low end settings that you monitor can't even render because it's at 160fps.