The SP rating is just based on the voltage/frequency curve. A common misconception is that a CPU core that needs less voltage is automatically superior to one that needs more. It is not necessarily true. Sometimes a core that gets by on lower voltage also has higher leakage, which means that...
I'm going to assume that we're talking about DDR4 and not DDR5. It doesn't make a big difference as long as you are running in dual-channel on a dual-channel platform or quad-channel on a quad-channel platform.
But, if you want to know what is ideal in terms of getting the last drop of...
That's been my experience as well. If a memory test fails, it doesn't necessarily mean there's anything wrong with the sticks. If I suspect a problem with RAM, it makes way more sense to me to just swap in known-good sticks and see if the problem goes away.
It depends. If the RAM is just going to be run at stock or XMP, then no. If it's some 3200-rated stuff that I intend to run much faster, then yeah, I'll dial it up and run some tests to make sure I know where I should draw the line.
Anyone who thinks that in 2 months a 4070/4080/4090 is going to cost what a 3070/3080/3090 costs right now is kidding themselves. You probably won't even be able to get one.