The best replacement for 6800 is 6600gt.

The best alternatives to 6800 are rated as following:
★★★★★  6600gt
★★★★★  x800
★★★★★  7800
★★★★  force 22
★★★  9000
★★★  7600
★★★  6700
★★  6800gt
★★  6300
★★  6700 ultegra
★★  8400
★★  7300
★★  6600
★★  6900
★★  6500
★  9800
★  6400
★  8600
★  6710
★  f6670
★  6502
★  radeon x800
★  6740
★  x850
★  geforce 7800
★  ati x800
★  6700 brakes
★  ati x1650
★  7900
  6800le
  8400 gs

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Speaking about 6800, one probably shouldn't forget about 6900.

Other search terms related to 6800 and their ratings are:
★★★★★ 6900
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6800
6600gt
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