There are three main generations of Mac Pro machines, each generation has particular RAM types and installation procedures
The models from 2006 to early 2008 use FB-DIMMs with large heatsinks in 667 MHz and 800 MHz. These models have two removable memory riser cards named A and B, with four sockets on each. The sockets are numbered 1 to 4, starting at the connector edge of the riser.
The rule for these machines is to fill the RAM in matching pairs, starting with largest memory modules in the bottom (A) riser card slot 1 and 2. Then the upper riser, slots 1 and 2.
A1 & A2
B1 & B2
A3 & A4
B3 & B4
The reason for installing in this order is that the 3 and 4 slots are marginally slower than the 1 and 2 slots, so you want the majority of your RAM in the 1 and 2 slots.
The models from early 2009 to Mid 2012 use DDR3 memory, 1066 MHz or 1333 MHz ECC DIMMs. The machines have the memory slots on the processor tray, they may have one bank of four memory sockets or two banks of four totalling eight, depending on if you have one or two Xeon processors.
With four socket machines, fill the memory in order from slot 1 through slot 4
With eight socket machines, it depends how many memory modules you are installing. Aim to use matching modules in a bank, you can have different sized modules in the two banks.
2: Bank 1 slots 1&2
3: Bank 1 slots 1,2 &3
4: Bank 1 slots 1&2, Bank 2 slots 5&6
6: Bank 1 slots 1,2&3, Bank 2 slots 1,2&3
8: All slots full
The highest performance is a set of three matching modules in each bank and the fourth socket empty. This allows the machine’s memory controller to use Triple Channel memory access mode.
The largest memory modules, 16 GB, cannot be mixed with any others, as it is ECC Registered memory.
The Late 2013 model uses DDR3-1866 MHz ECC DIMMs
There are four memory sockets numbered 1 through 4. Fill the sockets in numerical order. Apple does not suggest mixing sizes of RAM “Use the same size memory modules across all slots to maximize performance.” But they don’t come right out and say “don’t mix sizes”. With other Xeon/DDR3 machines, the memory controller can address RAM in groups of two (Dual channel) or three (Triple channel) for higher speed of access. If you mix sizes of RAM, the controller would have to shift down from Triple to Dual, or Dual to Single channel mode, which will lose about 6% – 8% in overall performance.
The Mac Pro Late 2013 model can take either Unbuffered ECC memory or Registered ECC memory, but the two cannot be mixed. Most MacPro memory is Unbuffered, but available 16GB modules are all Registered, so you cannot mix the 16 GB memory modules with any other size.
Apple has installation guides are here for
Gen 1 MacPro1,1 MacPro2,1 and MacPro3,1 machines 2006 – early 2008 http://support.apple.com/kb/ht4433#4
Gen 2 MacPro4,1 Early 2009
http://support.apple.com/kb/ht4433#3
Gen 2 MacPro MacPro5,1 Mid 2010 and Mid 2012
http://support.apple.com/kb/ht4433#2 , http://support.apple.com/kb/ht4433#1
Gen 3 MacPro6,1 Late 2013 (tubular)
http://support.apple.com/kb/HT6054