Maximum memory config for Mac Pro 5,1 (mid 2010)?
That machine will take 16's - my 3,1 does, to an unofficial maximum of 64GB [or more, according to Everymac, though I've not tested that myself].

The arrangement is not what you would initially think logical.


If you install equal-size DIMMs (for example, all 1 or 2 GB DIMMs) in your Mac Pro, Apple recommends that you fill the slots in the order listed in the table.


Note: If you install different-size DIMMs in your Mac Pro, follow the order in the table. If the DIMM configuration you install doesn't provide optimized performance, the Memory Slot Utility appears onscreen and recommends an improved configuration.



Source: Mac Pro (Mid 2012 and earlier): How to remove or install memory … &amp; previous similar confusion when I upgraded mine ;)