Installing Ram iMac 27''

Hi there,

I am going to purchase the newest model iMac 27'' and i need to know how to install extra Ram and what type of Ram i need?

I want to install 2x 16GB as soon as i have the iMac as i don't want to pay the extra for Apples Ram.

Here are the specs of the machine.

Link - Apple Store link

3.4GHz Quad-core Intel Core i7,

Turbo Boost up to 3.9GHz

8GB 1600MHz DDR3 SDRAM - 2x4GB

1TB Serial ATA Drive @ 7200 rpm

NVIDIA GeForce GTX 675MX 1GB GDDR5

And also just another question, How much better are the fusion harddrives?

Thanks