Well in theory Crossfire with a 290X would also work since they're the same GPU core, the 290 just having some disabled compute units, but the 290X would just get bumped down to 290 performance so you'd be paying extra money for nothing. You also need a pretty beefy PSU for 290 CFX (1000W+). So as previously said, much better to sell it and get a GTX 1070.