the 760 is a solid card, there are some upgrades over the 660ti its replacing. for example a better fan curve, its quieter and doesnt ramp up and down constantly, also it has GPU boost 3.0 which would basically let you tell the card " as long as you stay under X temperature, overclock your self as high as you want ". but as for real world performance difference over the 660ti, there is about a 5-10% increase in performance. However, the 4gigs of ram is " too big for this cards britches ". the only way you will be able to use all of the 4 gigs is if you are gaming in above 1080p resolution, in which case, you need to invest in a card that has more GPU processing horsepower, like a 780+. the 760 is an awesome card for 1080p gaming. If thats the resolution you are going to run at, get a 2gig card and call it a day!
take a look at this video, it will really help you understand what makes a good card:
hope that helped!