My 2p: you'll notice improvement as 660Ti is mid-range card and ensures you with good quality for a budget, but R9 290 is (in the mid of) high-range card delivering almost full fluency (no frampedrops under fluency level = 60Hz).
http://www.anandtech.com/bench/product/1038?vs=1068
(760 is something like 660Ti, but has more ROPs which gives a bit better performance with AA=on). More VRAM will do own job as well. So good choice (I've got 660Ti as well BTW), but don;t forget to clean up system after upgrade (download a DDU and use it before installing new nVidia drivers).
PSU is to be changed, one calls it "cooler disaster" for some reason

Antec, Bequiet, Corsair, Enermax, Sesonic, SuperFlower, XFX etc.