Basically:

6870 = 5850 + ~5-10% (In fifty frames per second that means a 2.5 fps increase. 50, to 52.5 or 55 for dumping $250. Astonishing.)
6850 = 5830 - 2 to 5% (Specs just plain suck IMO, it's not worth $199.)

Still, some of you are wondering why people are complaining about it.

Because they are most likely slower than the 5850 and 5870. Of course, if they aren't, it's because they hiked up the clocks.