iMac '08 with MacbookPro '08 memory. Different MHz. Possible?

Discussion in 'iMac' started by eyeangle, Jan 4, 2014.

  1. eyeangle macrumors regular


    Jan 2, 2014
    Melbourne, Australia
    My iMac 20-inch early 2008 uses 800MHz memory. (Has 2GB)
    My dead (logicboard) MacBook Pro early 2008 uses 667MHz memory. (Has 4GB)

    I've replaced the iMac 2GB(800MHz) ram with 4GB (667MHz) from the macbook pro into the iMac and it seems to be running fine. Can I expect any problems from this change is MHz?

    What's the difference between 800MHz & 667MHz memory?
    What is the limit of MHz memory I can put in the iMac?
    Is 2GB(800MHz) faster/better than 4GB(667MHz)?
  2. jav6454 macrumors P6


    Nov 14, 2007
    1 Geostationary Tower Plaza
    Yes, memory speeds can be mixed. But a faster FSB will run slower if the memory placed is a slower one.

    Example, say you put in your iMac 667MHz memory (iMac is rated for 800MHz FSB); then your FSB on your iMax (due to memory limits) is 667MHz. The memory will work fine and you'll be able to access it all, but performance will suffer.

    However, putting a faster memory into a slower FSB won't do a thing. Yep, tragic. Well it will do one thing, the faster memory will run as fast as the main boards FSB allows (which if you put 800MHz memory in a 667MHz FSB computer, the memory will run at 667MHz).
  3. eyeangle thread starter macrumors regular


    Jan 2, 2014
    Melbourne, Australia
    Ok great thanks. So, I'm thinking of upgrading the RAM to deck it out with 800MHz because I like the extra speed and I think I can tell the difference.

    I'm buying from here:

    But can you tell me this? From the site it supports up to 6GB memory in the iMac 2008. That's a 2GB and 4GB dimms. I'm thinking this option is the best.

    However, from it says this iMac only supports 4GB(2x2GB dimms) and I've heard that keeping dimms the same speeds/sizes (pairing them) will increase performance. So will having non-matching 2GB and 4GB dimm actually make a difference compared to 2x2GB?


    ...and it's a huge difference in cost, $60 compared to $140 for the extra 2GB
  4. Twimfy macrumors 6502a


    Sep 11, 2011
    You can use 6GB however the 4GB module will only go in one of the slots for some reason. The performance hit from having mismatched sizes will be so tiny you won't notice.

    As for the memory speed. My 2008 iMac will not accept anything slower than 800Mhz exact. I tried it with 667Mhz and it just wouldn't boot (although I didn't have a way to be sure that the ram module was good and working but I assumed it was).

    My 24" has 6GB of DDR2 800Mhz and runs fine but my 20" is only running 4GB due to the fact that I couldn't find a single 4GB stick that wouldn't require me to re-mortgage my house, to be honest though, running Mavericks it's as speedy as I could ever want it to be so I don't feel like I'm missing out.

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