For the most part it's usually best to just go lowest that's stable, there are some "rules" online but most of them are just random things someone came up with.

For the most part it's usually best to just go lowest that's stable, there are some "rules" online but most of them are just random things someone came up with. There are a couple JEDEC ones but the memory controller will adjust to work with those or it'd just not work. A 1:2 sync usually starts being worth it above ~7200MTs but I don't know how viable that'd be with 4 dimms, at the very least it'd need tuning the various termination impedances