Upgrade my RAM? #2

Hello, I am thinking about upgrading my computers RAM. Here are some system specs.

http://img822.imageshack.us/img822/968/capturerp.png

http://img180.imageshack.us/img180/9865/capture2ll.png

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This is my first and main computer (for gaming, browsing, and word processing) and would like to play Starcraft II, Dragon Age Origins, and Fallout 3 on it with medium-high settings. As it is now, only low settings is possible.

Will upgrading my RAM increase performance?

Should I upgrade my RAM, as in, is it a 20%-40% gain in general performance?

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Changing from a 32bit to 64bit OS is not an issue, but money is. Deciphering from the website below, would 4GB or 6GB be a better deal and will the 6GB provide a significant gain over 4GB; as I am willing to get either if it's worth it?

http://www.crucial.com/systemscanner/viewscanbyid.aspx?...

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Thanks much in advance everyone!

P.S. This is my first computer, the other post is about my second (and not so great) computer.

EDIT: Hey also, i'm going to make a post in the video/graphics card section for some help on that. I'll link it up when it's there.

EDIT#2: Here it is http://www.tomshardware.com/forum/292532-33-upgrade-gra...