The R7 290 is a DDR3 card, so I doubt it would be very efficient at mining Ethash, which is a memory intensive algorithm. There is a R7 290X card that has DDR5 memory and supposedly gets ~15 MH/s. ZEC would probably be a better option for that card.
http://forum.ethereum.org/discussion/comment/19394/#Comment_19394IMO, you would be better off going with a RX 560 or 1050 Ti cards, that get ~14 MH/s on ETH and are much more power efficient.
R7 290? Where you found such a card? And why respond about it when original poster asked for r7 240?