Crossfiring two ATI cards in most cases renders one of the cards useless due to software issues.
Both the cards do not scale well and almost all the times you see Black Screen and Micro - Sluttering as well.
For using more than two cards ATI is not the way to go.
GTX 760 as said is a very good option in my opinion. You also have HD 7970 if you can stretch your budget by a little.