Regardless if you could CrossFire an R9 290X with an R9 290, I wouldn't do it now. Even if you could get an R9 290X out of upgrading the BIOS for your one card that appears compatible, both cards will have to run with the lower settings between the two during CrossFire, so it would be pointless to your CrossFire configuration to improve the performance of a single card of the pair. I would only consider the risk of flashing to an R9 290X BIOS if you were planning to run that card by itself.