Re: Upgrading my computer parts
What are your future needs for this computer? That would dictate how much ram you need. If you are looking to game, add 8GB, if you are heavy on the video/graphic editing side of the house add 16GB if your motherboard can house it and if it is just for daily use (watching videos, streaming media, surfing the web), as InventD mentioned, just add 1-2GB RAM.
Also, if you are gaming, say with older games that aren't as demanding as some of the newer ones on the market, look at upgrading your GPU (Video Card) to say...a GTX 460/480. They are cheap, being a generation prior to the current market and will boost your performance.
I am going to stop there since I don't know much information about what you are looking for, because I will end up catching myself doing a complete overhaul on your system. Haha.