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):
- 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