It depends on the games you play and the games you want to play.
If the card you have now isn't playing the games at a rate you like, look at the charts and compare the numbers.
The belief is that the gtx260 is more powerful, but can't play dx11 games. On the other hand, it doesn't make sense getting a less powerful card.
Buying a mid range card now is tough as there are alot to consider. Look at the charts and buy the card that plays your games the best.