Not really much advice as graphics cards are pretty much 'horses for courses' and I am not really a games player but I would think that your processor is also becoming a brake on new game play. Often the system requirements quoted on the box of a game are optimistically underestimated. Also some modern cpu's are up to four times the speed of your present one.
It is all a sad story and it is called progress but don't spend too much on a graphics card alone. If you do go that way get a card with at least 128 Mb of DDR memory but don't spend too much. I have one with an Nvidea chip and have been very pleased with it although it is already out of date.