I'm in a position to get a new build now!

Who would you recommend for a pre-built machine (just looking at options, I may yet self-build)?  PCSpecialist?  Scan?  Overclockers? 

I can't remember how it works for overclocking RAM without the CPU, isn't that FSB x multiplier? Are you using XMP profiles or something? If the memory is rated to 3200Mhz and you're not adjusting the CPU then it should run fine at the higher speeds. Memtest86+ is still a classic for testing RAM stability (and that there's no errors).

Also worth noting that tighter RAM timings are probably more important than frequency.

I've installed the RAM now (correctly seated? third time's the charm, eh! ) and it's POST'd just fine.  It is running at 2133 which the highest published supported speed of the CPU.  Previous install ran 2x8Gb (matched pair) of Kingston at 2666 though so should I o'c it in BIOS to 2666 and see what happens?  I'm guessing the worst that can happen is instability - or worst case simply not POST? 

What does the BIOS setting do though - does it tweak the motherboard (which easilly supports it at that speed anyway) or is there some other CPU-based setting I need to fiddle with?    New RAM is 3200Mhz btw.

For DDR5 you definitely want a matched pair but not sure about DDR4 as I've skipped that, you are probably just as well getting a quad channel kit instead of two matched pairs if there's no price difference in it.

Why don't you use CPU-Z or the like to find out what spec RAM you have right now, then just buy bigger of the same?

Uh-oh.  Going down the rabbit hole here. 

Motherboard allegedly supports fast RAM but I've read that the CPU will only support a max of 2133Mhz DDR4.

Task Manager reports however that I'm running 2666Mhz so what should I do?  I'm just gonna get 2 sticks of 16Gb (as it is only an interim measure and I don't want to blow a hole in my wallet) but I'm assuming 2x 16Gb @ 2666Mhz will be 'safe' as it appears to be currently stable with that speed?