While that cpu is starting to show its age, I feel whats really holding you back is that graphics card. You probably arent even using 3/4 of that cpu's potential. A new $100-$125 video would blow away what you currently have, and if you still arent satisfied with your results, it could be transfered to a new system at some point. Any of these would be a nice upgradehttp://www.newegg.com/Product/Product.aspx?Item=N82E16814150540http://www.newegg.com/Product/Product.aspx?Item=N82E16814127490
Also look at this chart I will link to. It compares all the different cards for gaming to each other. The recommendation is to go up at least 3 levels above your current card, otherwise the diference may not be so noticable.
What power supply do you have, I believe these require about 450-500 watts and a 6pin power connector