Is it worth to switch one card for other and pay 70 dollars more for gtx660
the 660 is better by about 20% but i don't think that would justify the 70$...no i would not do that.
You will not really see that much of a boost in games we're talking 5-6 fps most of the time.
a considerable uprage to a 560ti would be a GTX670 or even a GTX760...there you would notice
a pretty good jump in performance that would be worth it...