To add some specifics for lemon’s benefit, 8000+ is with 1DPC 1R, AMD’s 1DPC spec is 5600, the highest rated 64 GB UDIMMs I’m aware of are 6400, the mobos I can think of that have 1DPC 2R specs claim up to 6400, and there’s little to negative returns on clocking higher than 5600 even if you get pretty lucky and lottery a dual chiplet part that’s stable at 2133 FCLK.
Personally what I do for 1DPC 2R tighten at 5600 and call it good. With 48 GB DIMMs at 5600 1.25V active cooling’s not needed and there’s plenty of thermal margin to tRFC2. So I suspect 64 GB’s likely to be fine with decent airflow unless ambients are pretty high.
Nope. All evidence is Promontory 21 lacks an x8 PHY and it’s easy to see from the X870 and X870E Nova’s specifications and block diagrams neither has an x8 slot. (Also ASRock’s dropped PG out of the board names this time around.)
If you want x8+x8 switched PEG from ASRock’s current production that’s X870E Taichi [Lite]. If Wendell’s on to something and ASRock hasn’t just caught up with today’s BIOS releases maybe X870E Carbon would be worth it.