Im thinking about buying a 27" iMac and on the aussie apple store it costs $220 to upgrade from stock 8GB to 16GB RAM. I've heard it is easy to find cheaper RAM and self-install, but I'm having trouble sourcing cheaper third party RAM. Apparently mixing different RAM is less reliable? And not all third party RAM is guaranteed to work in the iMac? So is $220 to upgrade with tested, reliable RAM a better option than DIY? Does anyone know exactly how much cheaper it would be to buy third party RAM in Aus?