A graphics upgrade is always good.
But how much better will be determined by your processor and the types of games you play.
Sims, mmo and strategy games may be limited by the processor.
For fast action games, the graphics card is all important.
Try this test:
Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.