As others mentioned before, the realistic expectation for any R9 7950X or R9 9950X would be DDR5-6000 for 2x48GB. DRR5-6200 means you are lucky with your CPU's memory controller, and DDR5-6400 is pretty much winning the silicon lottery. I've seen some people posting ZenTimings screenshots of DDR5-6600 at some extreme overclocking sites, but w/o an intensive stress test, I've got my doubts.
Usually, you want to run MCLK:FCLK:UCLK in a 3:2:3 ratio for the lowest latency when going for 6000MT/s to 6400 MT/s. Slight FCLK overclocks, anything between 2000MHz and 2200MHz, should be fine as long as Vsoc is reasonable (1.15V - 1.2V), since the infinity fabric uses error correction and re-transmissions. There are a couple of ways to test fabric stability, and it seems the most popular way is to play music in the background while hammering their CPU's memory controller while stress testing with something like linpack.
Alternatively, you could try to run DDR5-8000, at a 2:1:1 ratio, but that means you need a good and quite expensive memory kit with 24Gb Hynix ICs, and a motherboard on an 8-layer PCB that ideally only comes with two DIMM slots to get there without much trouble. From what I've seen so far, R9 9950X is still a pain when it comes to DDR5-8000 on 4 DIMM slot boards, but unlike the 7950X, it might get there eventually according to AMD.
Keep in mind that DDR5-8000 should actually benefit much more on dual-CCD chips while running heavy workloads than what most folks test these setups for. A 7800X3D won't see much improvement in gaming, since it only has a single fabric link, compared to the two of a R9 9950X.
From my own experience, I run a couple of R9 7950Xs and R9 7950X3Ds, mostly with 96GB of DDR5-6200 actively cooled at manually tuned timings. Usually a 80mm or 92mm cooler strapped onto the back of the GPU is fine, but if you want a nicer look, you can get a Jonsbo NF-1 as well. All sticks are either Kingston or G.Skill and use 24Gb Hynix M-die. Everything is stress tested for about a week running a mix of FurMark plus a collection of the usual memory intensive workloads, e.g. Prime95 (large FFTs), y-cruncher N63 & VT3, linpack Xtreme, GSAT via WSL2, etc.
One 7950X boots DDR5-6400 easily, but it's sadly not stable. Another one won't even post at that speed... aka silicon lottery.
For 128GB, I was able to run DDR5-5600 on a B650E Aorus Master on XMP/EXPO profile #2 with two pairs of Kingston DDR5-6000 CL36 kits out of the box. Those kits are Hynix A-die and easily clock to DDR5-6200 as well. Since that is my main editing machine (DaVinci Resolve, Twinmotion, UE4 & UE5), I switched them out for 96GB for the faster speed. However, with a current AGESA, it should be possible to run those 4x 32GB sticks at a 4:3:4 ratio, which should give them a bit more throughput due to the slight FCLK overclock without increasing latency. Right now, I'm waiting for another 8-layer PCB motherboard to try the 128GB setup in one of my render nodes, where memory speed isn't that important, but I'm still curious about silicon variance of the same type of AMD CPU.
Maybe I'm just lucky and got some golden memory sticks, but 128GB seems to be possible at 4800MT/s out of the box on even a 6-layer PCB board like my MSI B650 Carbon WiFi with a mediocre CPU.
The general problem with quad-rank or 4 DIMM setups is heat dissipation. Those above-mentioned Fury Beast sticks come with thin heat spreaders that allow just enough air circulation between them. My 48GB Fury Renegade sticks come with thicker heat spreaders and almost touch each other in the 192GB config. My G.Skill Ripjaws S5 kits are better in that regard, but none of my boards like their XMP profile and the PMIC lacks a thermal pad, which means that those sticks require a very high tRFC of ~190ns compared to the ~160ns of the Kingston kits to avoid overheating.
If you don't manually tune your memory, you can just use XMP/EXPO profiles and set a longer refresh interval with a shorter refresh cycle as the easiest way to noticeably overclock DDR5. However, you should still stress test it, like any other overclock.
