I'd like to add some aftermarket RAM to my Mac Pro (2013)

16GB and larger DIMMs are registered (i.e.., have a Buffer Register) they will not play nice with unRegistered DIMMs. So you need to decide NOW whether you are ever going bigger than 32GB, because doing so will involve 16GB and larger Registered DIMMs.

Buy from a Mac-Centric vendor Vendor who says:

"It works in your Mac,

Or your money Back."

Otherwise, you run the risk of owning some very nice DIMMs that work great, but do not happen to work in your Mac.

Of course you cannot locate these DIMMs in a local PC shop. These are Server DIMMs.

The larger the memory you install, the More important having the extra error-correction bits becomes. The Xeon Processor generates eight extra check bits on each Write, and check these bits on each Read. Single-bit errors are corrected on the fly in one stretched memory cycle. Double-bit errors are detected, and Halt your Mac on a kernel panic, machine check to avoid poisoning your data.

If you have ever tried to debug illusive computer problems, that may also be caused by marginal memory, the confidence of going forward saying, "It is NOT memory because my Mac has error correction" is just amazing. Trying to defeat this feature is folly.

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this site has very good service, and has always served me with the correct modules, first time every time:

http://www.datamemorysystems.com/apple-mac-pro-12-core-intel-xeon-e5-2-7ghz-md87 8ll/a-cto-late-2013-memory-upgrades/

I have no relationship with them except that of loyal customer.

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This site is among the most popular of all Mac aftermarket vendors:

http://eshop.macsales.com/shop/memory/Mac-Pro-Memory#1866-memory

you can trade in your old modules and save a little money. This is the rate at this writing (subject to change):

Trade-in Values for Memory from 1866MHz DDR3 DIMM Mac Pro 2013 systems:

• Apple Original 4GB - $17.00
• Apple Original 12GB Set (4GB x 3) - $51.00
• Apple Original 16GB Set (4GB x 4) - $68.00
• Apple Original 8GB - $32.50
• Apple Original 32GB Set (8GB x 4) - $130.00
• Apple Original 16GB - $80.00
• Apple Original 64GB Set (16GB x 4) - $320.00

I added to my post after the time of your reply. be sure to check it again.

I used to simply suggest against installing non-ECC RAM, telling Users that defeating this excellent feature was a really Bad Idea.

But recently, I have been unable to even FIND DIMMs this fast that were not ECC. You are considering Trashing one of the best features of your Mac Pro to save what, US$20?? Forgetaboutit.

Why slow down your Mac by installing slower RAM? RAM backed by Vendor support is available at reasonable prices, and includes a LifeTime Warranty.

1@8GB at Apple: US$200/each

1@8GB at DataMem under US$80/each

1@8GB at OWC/Macsales Under US$90/each, under US$80/four, and then you can trade in on top of that.

But also consider substituting 2@16GB Registered for about the same or Less, and keeping two slots open for future expansion.

These forums are paved with Users who bought RAM that seemed like it should work in their Mac, but does not.

I ended up posting advice about Mac-centric vendors so many times, one day it condensed into that rhyme I posted above.

OWC sells 32 GB stick for the 2013 Mac Pro and those are 1333 MHz

http://eshop.macsales.com/item/Other%20World%20Computing/1333D3Z3M096/

Does the vendor say the 1600 MHz sticks will work in your Mac or offer a free refund?

What Vendor did you use in Australia?

Did they recommend these DIMMs? If not, how did you decide on these specific DIMMs?

Do they come with money-back guarantee?

Do they come with lifetime warranty?

It sounds like you reached some good compromises that work for you. Congratulations.🙂

When I looked on the Kensington web site they had 11 different 16GB DDR3 ECC 1866 modules. When I looked at slower modules, there was no guidance about which might "downshift" and work correctly in a Mac Pro (which is one problem some Users have faced, among others).

I am glad you were able to weigh the costs vs the risks and come to a decision, and that you won the bet that these would work. Other Readers in a similar situation will thank you as well when they read about your choices and learn more about this complicated subject.