Apple - Is it ok to mix 2 GB and 4 GB DIMMS in an iMac (mid 2011)?

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.

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.

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.