I've got an early 2008 Mac Pro with 4 Gb of memory which I would like to upgrade, but I'm unsure of the memory speed. In my "About This Mac" dialog it says the memory speed should be 667 MHz, but online (even on Apple's own site), it says 800 MHz. Is it okay to get 800 MHz memory and mix the two, or should I hunt for 667 MHz memory? Or replace my existing 667 MHz memory with 800 MHz?

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4 Answers 4


The difference between 667 and 800 is negligible at best. You will not perceive any difference at all in your daily tasks regardless of what those may be, the one exception being if your job happens to be running theoretical benchmarks all day. You will get a bigger performance boost from populating all 8 slots than you will the performance decrease from having 667 in there. The price premium for 800 makes it a ridiculously poor decision, dollars/performance.


According to Everymac, your Mac Pro takes 800 MHz D DDR2 RAM. I'm not sure why your Mac is reporting 667 MHz, but try using the Crucial Memory Scanner and see which they recommend for your specific system.

As far as mixing RAM, it can be done, but it isn't really recommended. From a question on Super User:

When different speed RAM modules are used, all of the RAM should clock to the speed of the slowest module. Many times, I've found that mixing RAM speeds sometimes causes system instability. I've had plenty of cases where RAM sticks that tested just fine alone wouldn't play well together. So I strongly recommend that all of the RAM installed in your computer be of the same speed.

  • I'd go with the 800MHz ones then, not what your computer says. That's really odd, but I'd trust Apple's site, and Crucial's scanner…
    – daviesgeek
    Commented Sep 24, 2013 at 22:49

If the 667 MHz memory is what is currently in your computer, that is what will be reported, and it works just fine in the 2008 Mac Pro, however the Mac Pro supports 800 MHz and it will run faster in the event you decided to upgrade for memory read/writes (which can be a bottle neck and you should consider upgrading if speed is that important to you.)

I recently changed my RAM from 667 MHz to 800 MHz on my Mac Pro with the following results reported by NovaBench:

677 MHz speeds (1 card only): 4096 MB System RAM (Score: 104) - RAM Speed: 2028 MB/s

677 MHz speeds (2 cards populated): 8192 MB System RAM (Score: 146) - RAM Speed: 2132 MB/s

800 Mhz speeds (2 cards populated): 16384 MB System RAM (Score: 189) - RAM Speed: 2343 MB/s

I've seen speeds of up to to 2600 MB/s as well with 800 MHz RAM. I need to fiddle with the RAM and see why I am not getting those speeds right now, as I had been earlier.

What ever the slowest memory is in the computer is what the Mac will run all the memory at. So if you have 667 MHz memory and mix them, all of it will run at 667 MHz. Replacing the memory is the only way to get the speed increase.

  • Is there a noticeable difference? I've already shelled out for the 667 MHz memory, and the Mac is more stable with more memory, so I'm wondering if it would be worth it to change to slightly faster memory.
    – Lizzan
    Commented Jan 2, 2014 at 13:09

I eventually went with the 667 MHz RAM, because I expect my own computer to know what's actually in it more than I'd trust external programs or websites. So far everything seems to be okay.

  • Uhhh... You're Mac Pro is meant to use 800 Mhz memory. 667 Mhz works, but it'll be slower. manuals.info.apple.com/MANUALS/0/MA430/en_US/…
    – Alexander
    Commented Dec 31, 2013 at 2:21
  • @XAleXOwnZX Will it be noticeably slower? It's what was in my Mac when I got it, bought new. (Not directly from Apple, through a reseller).
    – Lizzan
    Commented Jan 2, 2014 at 13:26
  • Well you're using a Mac Pro... you paid a lot for a premium machine. You're polluting that large investment with slower memory
    – Alexander
    Commented Jan 2, 2014 at 17:56
  • @XAleXOwnZX Yeah, I PAID a lot, more than five years ago. Unless I would actually notice a difference, shelling out for at least 8 Gb of faster memory would be a waste of (quite a lot more) money.
    – Lizzan
    Commented Jan 5, 2014 at 13:56

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