Mac – Intel Mac mini – maximum RAM Upgrade

The difference in speed between matched/unmatched pairs will be completely negligible vs having more RAM.

Tell your friend to get as much as he can. 3 > 2 always :)

I haven't tested that particular case, but I have played with a Mac Pro (that offers eight memory slots) and although I have them in the "best possible scenario", when I didn't (for a while I was testing a different "less optimal" setup), I couldn't really notice the difference…

I recommend you head over to Crucial and fill the fields to find the type of memory you can install. They have a decent model/type database and tips to obtain the best combination.

But again, I recommend > memory rather than worrying about memory performance. More is better :)

This is down to a limitation in some memory controllers and how they handle memory sticks with different memory characteristics (also add into the fact that you have no control over these settings in EFI as you might have in a non-EFI BIOS)

When you put both new sticks in, the memory controller is trying to set up the memory according to the specs advised by the PNY SODIMMs, however it is having problems with that configuration and fails to initialise the memory.

When you put in a stick of the older RAM, it changes the configuration to use the slower RAM's settings and the second stick of RAM is actually running at the slower speeds and timings of the old RAM and these are invariably "slower" and more conservative than those the PNY sticks can run at.

The other posters are correct, the only solution is to get different RAM and try those. I would also advise to use a more reputable brand, the cost differences are minimal and the warranties are better.