Can I mix the two 2 GB RAM chips in my iMac 21.5″ (late 2009) with two other type of 2 GB RAM chips? I have not opened my RAM slots yet but Apple says I have two 2 GB DDR3 chips. I want to add two PC3 8500 chips to it. Will it hurt to try?
As a general rule, you can mix sticks of RAM as far as brands/speeds/timings/sizes.
Your computer should accommodate different speeds/timings by scaling down speeds/timings on the faster sticks until they match the slower sticks.
Brands will matter as far as they do for everything else: everyone wants you to buy their brand of stuff, but they'll generally work. I've stuck random brands into random Apple/Windows machines and the ones that aren't just terrible do always work.
Depending how far apart the specs on your sticks are, you may have stability issues, but those are easy to fix - remove the stick you added.
According to Everymac, that's what it takes...
RAM Type: PC3-8500 DDR3 Min. RAM Speed: 1066 MHz Details: Supports 1066 MHz PC3-8500 DDR3 SO-DIMMs (204-pin). Standard RAM: 4 GB Maximum RAM: 16 GB