And is the r9 290 is better than r9 270x crossfired?
http://www.tweaktown.com/articles/5984/sapphire-radeon-r9-270x-2gb-toxic-video-cards-in-crossfire/index10.html
From these benchmarks yes it is and 1 gpu is always more stable and reliable than 2.
The difference between the r9 280x and the r9 290 is the 290 gets 10+ more fps which can be a big difference. The 290 also holds up better at higher resolutions which means it will be more future proof.
And is the r9 290 is better than r9 270x crossfired?
http://www.tweaktown.com/articles/5984/sapphire-radeon-r9-270x-2gb-toxic-video-cards-in-crossfire/index10.html
From these benchmarks yes it is and 1 gpu is always more stable and reliable than 2.