Hello,
I am thinking of converting my old pcu into mining rig (very small ofcourse). I have a p5n-e sli motherboard 2 pcie slots, 550w psu,e6400, well ventilated case.
Currently I have one offer for graphic card PowerColor R9 290 4GB at 350eur , sapphire hd7950 at  240 eur or r9 280x gigabyte at 290e (european prices..).. Which would be better "bang for the buck" from the perspective khash/watt?
I'll be probably mining just vertcoin

Thanks!