Thoughts on upgrades

[MiRai]

RAM
I can go with 4x 8GB ram (Corsair Vengeance), for about the same cost.
This would be 1866MHz (10-11-10-30 timings) instead of 2133MHz.
But would also allow for a future upgrade of the same ram, for 64GB total instead of 32GB.
Does 2133 make that much of a difference? Which would be superior: 2133MHz at default timings or downclocking the speed, but tightening up the timing? Or is that even worthwhile?

Only in benchmarks. I might suggest going with some CL9 1600MHz 8GB modules from G.Skill but, if the price isn't right then the price isn't right. 1866MHz CL10 is probably just as fast as 1600MHz CL9 and it won't make a difference either way, but SB-E is only rated for DDR3-1066/1333/1600.

Also, don't buy anything besides 1.5V (or lower) rated RAM. Higher voltages are not recommended by Intel and they have the potential to harm the memory controller on the CPU.

Is there much of a benefit to 64GB over 32GB, for a 10-box gaming machine?

Only if you want to use part of that RAM as a RAM drive. They're nice and fast but, can also be tedious to setup; especially if you're constantly turning your machine on and off.

The Noctua NHD14 leaves me with 44mm clearance.

I don't know much about clearance numbers but I will say this, those Corsair modules with the crazy fins on them have been known to cause clearance issues on motherboards with larger aftermarket coolers. They did for me on an 1155 Gigabyte motherboard, and I was barely able to squeeze by with an 1155 Asus motherboard... so different motherboard manufacturers may place the DIMM slots a few centimeters closer or farther to the CPU socket than other manufacturers.

It's hard to know for 100% sure since I highly doubt those measurements are available to the public, and that's why you may want to start with only 32GB of RAM and check the clearance surrounding the CPU socket with your aftermarket cooler.

SSD
I already own the 2x Vertex 3's...
I see most people recommend Intel 510's or Crucial M4's.
I have a four year warranty on the OCZ's, instant product replacement (with brand new drives), and am a 15-min drive to my local store.
Is it worth going with 510's or M4's, over my Vertex 3's?

If you've got fully functioning SSDs, there isn't much need for a replacement seeing as they're still under warranty. If anything, I would say run a secure erase on them to bring them back to a the factory spec if they're feeling sluggish. OCZ should have a tool that does this or their forums should have a sticky with the necessary information.