First off let me say this right up front, I do not play games so my answer will be in theory only.
The fact that you say some high poly count games shudder leads me to think that your graphics card is reaching its limits.
Although ram is cheep right now, I think you would be better off saving the money towards a new motherboard, CPU and graphics card.
As newer games come out all the ram in the world on your current system will not help.




