you probably would want to look into getting a new gfx card, as that is your weak point in the system. Also upgrading to 1gb of ram would be a solid upgrade. another thing you might want to look into is a whole new computer. you're current rig is a bit dated, and for around 500-600 bucks you could get a solid rig that would perform most current games on playable settings for a long while.
also, welcome to TF, enjoy your stay
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