The current OWC 8GB modules now mix and match with 1GB, 2GB, 4GB OWC and Apple modules (formerly, the modules would not mix with other size or type modules).
As a practical matter, most users go with all 8GB modules (24GB or 32GB in a 4/6-core Mac Pro), but this advance does mean that the following example scenarios are now compatible:
- A Mac Pro with 3 X 4GB can now add one 8GB module for a total of 20GB.
- A Mac Pro with 3 X 2GB can now add one 8GB module for a total of 14GB.
- An 8/12-core Mac Pro with 6 X 2GB can add two 8GB modules for a total of 28GB.
- ... etc ...
Fastest memory bandwidth is 3 modules in the 4/6-core Mac Pro, and 6 modules in the 8/12-core Mac Pro. The difference is small, but measurable.
I recommend purchasing 1333 MHz modules even if the Mac Pro model uses 1066 MHz, since the price difference is trivial vs 1066 MHz module, and the modules become more saleable/interchangeable at 1333 MHz (should you ever sell them or change to a different Mac Pro model).
We are still supporting the other 'restricted compatibility' 8GB modules/kits for those with them. The price right now is very similar between the two types making for a good transition point.
Note - universal 8GB will not mix with the restricted type 8GB (or 16GB…) as the others remain restricted as per original/current spec info.