Upgrade Disk Space

Nothing is stored permanently in RAM, so once you shut down your computer it's cleared. It's as simple as taking off the back cover, removing the two old sticks and replacing them with the two new sticks of memory. Close the cover and start up the computer

As far as brands go, there are a lot out there. Some favor a certain brand, but you'll be fine as long as you stick with a major manufacturer. Personally, I have G.Skill in my MBP and have never had a problem. There are a lot who use Crucial around here, too. If you go to the Crucial site, you can choose your computer and they will give you a list of their memory that they guarantee to work

 

You can find and buy memory (RAM) on Amazon.com or Newegg.
You don't need to save anything when swapping RAM. Your data will remain on your HDD that is unless you are swapping the HDD too.

 

@OP

No offense, but you seem to be confused about the difference between system memory (RAM) and storage (hard disk, HDD, HD, or SSD). Under these circumstances, are you sure that you want to shop online, select the correct product and install it?

If you want to avoid Apples overpriced upgrades, you could still find an electronics store like microcenter or Frys (not bestbuy, that's a consumer store!) and get help with selecting the correct memory, and maybe also have them install it for a small fee. Just a thought...