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I've been told it is important to use matching DIMMs when installing memory to take advantage of the 128 bit throughput. I'm looking to buy a new 27" iMac (mid 2011) which has 4 memory slots. If I put matching 2x2GB in 2 of the slots and matching 2x4GB in the other two slots, will that reduce its throughput or cause any problems?

Do all 4 have to match or do each set of 2 need to match?

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Kenny,

To your first question, yes, you can mix memory sizes.

To your second question, 4GB of matching memory will be faster than unmatched memory, but 8GB of unmatched memory will be faster than 4GB of matched memory. At some point you stop getting gains from more memory, but that depends on what you're doing.

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Yes - it's OK to mix. The more RAM you have, the more files can be cached over time and the more processes can sit resident in RAM for faster switching between tasks running at the same time.

I haven't read the tech note on the latest iMacs, but in general I have never seen a benchmark that shows any mac ever ran slower with more memory. (even the initial intel GMA integrated MacBooks where having non matched RAM where there were only two slots did noticeably slow the graphics performance). The iMacs of the past several years all need high quality ram that matches their specifications fairly exactly - but run well with whatever lineup of good ram is installed.

If it makes a difference, the tech note and consumer installation guide will tell you in which order to install RAM and will call out when matching pairs will make sense due to the architecture of that machine. Again, more ram is always faster.

Also - when you are buying a new mac - often the seller will credit the value of the pre-installed RAM as new memory if you want to buy up to have larger chips (2x4GB) and then you can add the second later as needed.

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I understand that more RAM will be faster, but I want to make sure I'm getting all of the bang for my RAM buck. So, yes, 12GB is faster than 4 GB, but is 4GB of matching RAM faster than 4 GB of unmatching RAM? –  Kenny Wyland Jun 5 '11 at 16:36
    
I'm going to be buying the machine through Amazon, so I don't have the option to replace the RAM at purchase like I could buying directly from Apple. –  Kenny Wyland Jun 5 '11 at 16:39
    
I have not seen any benchmarks or real world experience where running a pair of 2G chips slowed down a pair of 4G chips. These machines will scream with 8G and you'll be running at 12. The i5 and i7 chips need quality fast ram - but as long as the ram you add is proper speed/timing/voltage etc... you will get the best bang/buck by using the 4G that came with the mac. Don't remove the 2x2 until you need more total ram than you can afford with it still installed. –  bmike Jun 6 '11 at 16:45
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Just to add to your peace of mind, my 21" iMac (prior version) is working beautifully at 2x2 and 2x4 (12 total) GiB. Way faster than before (4GiB really isn't sufficient for what I do), and I don't see any oddball slowdowns.

I got good quality memory that matched the MHz (1333) of the existing memory. Works like a charm.

That said, I didn't try going to 16 GiB, so I can't vouch that 16GiB isn't faster than 12, but for me, 12 is more than sufficient at the moment. When the time comes, I'll switch out the older 2x2 sticks, but I don't see that it will make a huge improvement.

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Yep, totally okay to do. All Intel-based macs support dual-channel memory, so in a nutshell: You'll get the best performance with equal-size RAM sticks, but it won't kill it if you don't. Luckily, most RAM is sold in pairs these days.

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