The best replacement for 7990 is titan.

The best alternatives to 7990 are rated as following:
★★★★★  titan
★★★★★  780
★★★★  290x
★★★★  690
★★★  780 ti
★★★  gtx 690
★★  7970
★★  titan black
★★  gtx 780 ti
★★  280x
★  6990
★  r290x
★  8990
★  770 sli
★  7970 crossfire
  r9 290x
  290x crossfire
  gtx titan
  680
  7970 cf

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Speaking about 7990, one probably shouldn't forget about 7970 crossfire.

Other search terms related to 7990 and their ratings are:
★★★★★ 7970 crossfire
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7990
titan
780