It is a very broad question with literally dozens of right answers. If the card you have no longer performs to your satisfaction, then you should upgrade of you have the money.
If the card is broken and outside of warranty, you should upgrade. This would be evidenced by artifacts in the video, game crashes resulting in a colored screen, glitching video, and many other things.
If you want to take advantage of DX11 or are still using a DX9 card, you should upgrade of you have the money.
If you are disatisfied with the particular brand of card you have, NVIDIA vs ATI, and would like the features the opposite offers, you might want to upgrade, but it would be a bit silly.