Just for the sake of my OCD, there's no such thing as a "6650" unless we're talking about the ultrabook processor (i7-6650U)... which we're clearly not.
With that out of the way, I argee with others, generally speaking the 6600 is not worth the price premium. It's 100Mhz (~3%) faster on the base clock and 300mhz (~8%) faster on the turboboost clock. So technically, in CPU limited situations it could be between 3 & 8% faster, maybe a few FPS at best. The thing to remember is that CPU limited situation are rare, especially with a nice Intel i5 like you have. So in reality, it will very occasionally, from time to time, net you an indiscernible FPS increase... thus NOT worthwhile.
Save the money or spend it elsewhere.
RX 480 is a good card. If you're planning an upgrade to 1080P it'll be worthwhile over a cheaper card like the GTX 960 in the long run.