Well, before you do anything, you have to determine how much RAM the motherboard can support. Not all motherboards can support how ever much RAM you want. Go here: Gateway Computer, Server, Portable, Consumer Electronic Product Support
and enter in your serial # and find the specification sheet and it should tell you the max RAM supported.
Then you need to find out the type of RAM. Like if it's 1333 DDR3 or 1333 DDR2 RAM. Then you can start shopping. Luckily (for you, haha) I found out the type of RAM and how much is supported.
8GB is the MAX and in order to utilize the full 8GB you need to have a 64-bit OS.
These should be good: http://www.newegg.com/Product/Produc...82E16820134192
Simply get 4 of them and you've got 8GB!