You can't compare 960 sli with 290 cf, it's totally different, the 290 is at a much higher level of performance than the 960.
Instead, you can compare a 970 sli with a 290 cf config, and my recommendation would be getting a 295X2 since its single slot, produces less heat than the 290's and is watercooled by default + the price difference isnt that much.