MacPro Memory

Mac Pro 2009 thru 2012 use triple-channel memory, so three matched modules give you maximum interleaving.

The fourth slot shares its address space with the third slot.

The incremental difference between different numbers of modules is small. If you go for four large modules, sometimes the fourth module can't be made to work correctly.

There is a break at 16GB and larger modules. Those have so many chips that ordinary parts can not work together, so an output Register is added. Registered and Un-Registered can NOT be mixed.

two 16 GB modules is a good choice. That puts you into the upper category, does not break the bank, and leaves some room for more if needed.

Yes, that is exactly how these were sold.

3 sticks provided best interleaving, which was slightly faster. Thorough testing done after release showed that there was so much caching in the processors, the very slight increase was very difficult to measure, except in a memory test designed to emphasize the difference.

Not only that, Apple got used to shipping three modules in four slots, and in the late 2013 models, which did Not have triple-channel RAM (but did have four slots) they where initially sold exactly the same way.

On the single-processor-chip MacPro models, Four does not always work, unless they are matched pairs and the seller guarantees they will work.

The most popular seller of guaranteed-to-work modules for these Macs is OWC MacSales.com.

They are currently showing ONE 8GB modules for US$22.99, or a matched set of four for slightly less, US$88.99. If you choose to add ANY more RAM later, ALL of these un-Registered modules must be set aside and not used.

If you instead buy 32 GB as two Registered 16GB modules, that set is being offered for US$63.99, and is expandable by adding at least one more module later, and possibly two if you get lucky.

Unless you were really cramped for RAM, and kept spilling inti=o Swap space, a RAM upgrade does not tend to change performance much.

¿ more SPEED?

You get that from changing to an SSD boot drive.

Those guys (and a few others) say:

"it works in your Mac,

or your money BACK.

It costs them if they get it wrong, so they make sure they get it right, and you get the benefit.

A lifetime replacement warranty is also provided, and the stickers on the modules are all you need for proof of purchase.