how big is your power supply? what games are you trying to play?
if your not sure and or its low wattage, the r7-260x would be a better choice as it is much lower power than the pitcairn based r9-270. otherwise the 270/x would be about as high tier gpu you would want to go and like said, in some demanding games you might experience some bottlenecking from the cpu, but not a deal breaker. as far as using the igpu in conjunction with another gpu, hybrid crossfire, it can be done but it causes more issues than its worth and only provides a relatively small performance increase, if any at all, in a few games.