A Vega 64 is about the best one can commonly acquire for such uses, yes. Radeon Instinct MI25 cards are 16GB Vega GPUs of the same rev. as Vega 56/64, "Vega 10". Using MI25s as a 'gaming GPU' is anything but a plug-and-play experience, however.
You're asking the wrong person about a 5700XT being faster than a Vega 10/20 HBM-equipped card.
(Vega 20 is Radeon VII, and we shall not speak of this card, just the GPU die. )
In all seriousness:
A 'top-tier' board partner and Vega 64 Liquid Cooled Frontier Edition is meeting-beating a 5700XT and a 'boring' Vega 64 is w/in spitting distance of a 5700XT.
With undervolt, overclock, and p-state tweaking; Vega 10 and its HBM2 will beat any (out of box) 5700XT. (perhaps, excepting top-shelf OC cards like the Red Devil).
The Fiji (Fury), Vega 10, and Vega 20 GPUs were really something else. Even when benchmarks may say they're obsolete, there's just something about 'em
Atop their impressive qualities they're a curio/relic already to collectors; we're extremely unlikely to ever see HBM GPUs in the consumer-facing market, ever again.
For gaming and many other User-level multi-tasking(s), yes; 64GB is more than sufficient, and faster is better (generally).
I doubt there's any 'tweaking' or 'OCing' options in the SuperMicro BIOS, and I don't even want to start researching BIOS mods/modding... So, fastest supported-JEDEC in just 2 slots would've been my choice too.
Similarly, I've accepted that 24GB RAM just isn't gonna happen on my K8N-DL. 2x4GB on ea. CPU is much easier to get stable. (BTW, It took 4 separate purchases of literal-lots of RAM to find 4GB ECC DDR1 that worked)