Q: Can I use different brands/ sizes of memory?

Yes, that will be fine. Just make sure the manufacturer says it will work with your Mac Pro. Macs are picky about memory.

Crucial and OWC are good vendors

it uses triple channel so I would jst stick  one in each bank, but the memory slot utility will tell you if it is wrong, the end two are a pair for each bank and should, and may need to be identical.

2 x 8GB on each bank would be a better and close to ideal.

not just mac memory but mixing is always a concern, buffered, unbuffered, registered, etc.

I have mixed sizes in my Mac Pro.

I am using two 4GB modules and two 2GB modules.  I placed the larger ones in the first two slots.

All modules are the same brand (Apple factory Hynix).  I've never tried mixing brands.

How To Install and Remove Memory Mac Pro

Mac Pro 2013

http://support.apple.com/kb/HT6054

Crucial 32GB (2x16)

http://www.crucial.com/store/listparts.aspx?model=Mac%20Pro%20(Late%202013)

Mac Pro 2009 - 2012

http://www.datamemorysystems.com/apple-mac-pro-quad-core-intel-xeon-nehalem-2-66 ghz-mb871ll/a-early-2009-memory-upgrades/

64GB in 2009-2012 single cpu running Mavericks

http://forums.macrumors.com/showthread.php?t=1701517

Difference between RDIMM and UDIMM

When you populate slots 3 & 4 you do take a hit as they share one channel.

If 2 memory DIMMs are used per channel, this "...results in a reduction of maximummemory bandwidth for 2DPC (DIMMs per channel) configurations with UDIMM by some 5% in comparison to RDIMM." ^[1] (p. 14). This is because "...when you go to 2 DIMMs per memory channel, due to the high electrical loading on the address and control lines, the memory controller uses something called a “2T” or “2N” timing for UDIMMs. Consequently every command that normally takes a single clock cycle is stretched to two clock cycles to allow for settling time.

Therefore, for two or more DIMMs per channel, RDIMMs will have lower latency and better bandwidth than UDIMMs." ^{[2]  http://en.wikipedia.org/wiki/Unbuffered_memory}

HI,

Theres no need to bin your memory just yet, but hatters is right, long term you want to change over to ECC Rdimms, they have bigger capacities and they good news is they are cheaper to buy.

You could upgrade with ECC Udimms buy you would be paying around double, you better off upgrading 24GB of Rdimms.

16GB (2x8GB) of ECC UDimms will cost you the same as 24GB(6x4GB) off ECC RDimms

Hope this helps

<Edited by Host>

Always test your new memories ALONE so that you know they are working. Once you know they waork,  add the others back in to see if all will "play nice together"

One 8GB DIMM is about the same price as two 4GB DIMMs, so you should buy the denser 8GB DIMMs -- they are a better value and use fewer slots.

Largest DIMMs go in the lowest-numbered slots on each side, or the memory configurator will yell at you.

add the others back in to see if all will "play nice together"

What a ridiculous approach, your advising someone to buy memory totally blind and THEN check if the memory modules are compatible.

You go on to use the word dense in the mext paragraph, you have absolutely no idea what your talking ablout, your totally out of your depth pal.

STEP 1. Find out if your running ECC UDimms or ECC RDimm

STEP 2. Decide what path you want to upgrade down, UDimms or RDimms

STEP 3. Buy memory thats is guranteed to work with your setup (dont leave it to luck as someone suggested)