Oke I had the opportunity to test 2 cards so I made a comparison of them using this benchmark.
The test is done at the system you can see in my signature accept of course the videocards the changed.
both cards I didn't run at stock settings, but I overclocked them both with +50 for the core and +100 for the memory.
The power limiter was set to the max +50
The first card I tested was the sapphire r9 290.
This was the score of that card.
The second card I tested was the gigabyte r9 290x, The score of that card was,
So the x will give you a nice boost, is it worth the money?? That is something you have to decide for your self.