The 7990 is known for its micro stutter. It is cheaper and faster than the Titan however. AMD has greatly reduced the micro stutter of the 7990. But honestly if you are going that route, I would get the 7970 ghz in crossfire. I currently have the 660ti, and since I can't SLI, I am either doing that or getting a 780 (which OC'd is just as good if not better than the Titan).
But the 7970 crossfire is a much more cost effective choice and almost as powerful. A 7990 is just two 7970's anyway.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125439&nm_mc=OTC-pr1c3grabb3r&cm_mmc=OTC-pr1c3grabb3r-_-Video+Cards+-+AMD/ATI-_-GIGABYTE-_-14125439
But the 7970 crossfire is a much more cost effective choice and almost as powerful. A 7990 is just two 7970's anyway.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125439&nm_mc=OTC-pr1c3grabb3r&cm_mmc=OTC-pr1c3grabb3r-_-Video+Cards+-+AMD/ATI-_-GIGABYTE-_-14125439