Upgrading problem with 2012 Mac Pro

The 2012 model Mac Pro silver tower can use 1333 DIMMs, but will only ever see a speedup when ALL are that speed. The Quad processors do not take advantage of the faster speeds at all, but can use 1333 DIMMs as if they were the slower speed. [MacBOOK Pro can NOT use faster speeds at all.]

As you already have done, you can upgrade these DIMMs by the each. There is a very slight speedup when you attain matched triple, but that is usually overwhelmed by having enough RAM available.

All 16GB DIMMs have so many chips on a DIMM that they will not work unless an output Register is added to each DIMM. The output Register is so much stronger than chip outputs it overwhelms regular "un-Registered" DIMMs, so the two types can not be mixed. If you add any 16GB DIMMs, smaller, unregistered DIMMs must be removed.

Old versions of some 8GB DIMMs are sometimes Registered, sometimes not.

This page show what is available from this leading Vendor in the US. They also do research in their own labs to try to push the envelope. This company prides itself on customer service, so they simple refuse to offer what is troublesome. The charts show that 8GB DIMMs are available, in both 1066 and 1333 speeds, and theirs are un-registered.

Mac Pro Memory Upgrades Up to 128GB

It will cost as much again to ship from the US, but this will show you what should be available worldwide, if you search hard enough.

Note that certain end-point cases do not seem to work, such as four 16GB Registered DIMMs in the four-core or six-core models.

That vendor is a very safe choice. Note that you should get matching 8GB DIMMs, at the same time. Buying from different sources might result in some DIMMs not being recognized. I ran into this problem when I added a 4th 8GB DIMM to my Mac Pro, a couple of years after installing the others. Same vendor but the new 8GB DIMM did not match the other three from several years back.

You do not need to order two or four, your model Mac does not require matched pairs like the 2006 through 2008 models. You can upgrade by each DIMM.

4 DIMMs of 8GB (the one you circled) is a configuration they claim it will work.

The end point case I was referring to is the item directly below your circle -- if you choose 16GB modules, and four of them, they do not think that will work in your 4-core Mac Pro.

If you were considering a later processor upgrade (to a 6-core processor), you could also use 1333 DIMMs from the second chart, but they will not run any faster today, only after a processor upgrade.

1) [add some now, and ] if it works , then I can always add more ....

that is what I would recommend.

2) [will it really support 32GB? do I have to buy all the same to be sure?]

it will support up to 96GB up to 10.9 Mavericks, up to 128GB in 10.10 Yosemite or later. You don't NEED matched sets, but everyone will be happy to sell you matched sets, because they make more money that way.

3) [Can it really handle faster and a lot bigger?]

Your Mac CAN run with the next faster speed, 1333. But with the quad-core processor, it will see these faster DIMMs as the "ordinary" 1066 DIMM, and will not try to run them any faster.

Faster DIMMs (if they actually cost extra) are a waste of money -- buy the slower ones. Faster DIMMs you buy now would only help if you thought next month you might upgrade your Mac to a 6-core processor. The 6-core processor will run the 1333 DIMMs at 1333 (but the 4-core can't do that, it always runs them at 1066.)

Mac Pro WILL accept faster DIMMs and use them without issue (possibly at slower speeds).

MacBook Pro requires the EXACT correct speed -- not faster, and not slower, or it crashes a few times a day.

what processors in your Mac Pro silver tower?

what markets are readily available to you for memory purchase? US? EU? UK? Australia? Singapore?